draft-ietf-rtgwg-mrt-frr-architecture-10.txt   rfc7812.txt 
Routing Area Working Group A. Atlas Internet Engineering Task Force (IETF) A. Atlas
Internet-Draft C. Bowers Request for Comments: 7812 C. Bowers
Intended status: Standards Track Juniper Networks Category: Standards Track Juniper Networks
Expires: August 8, 2016 G. Enyedi ISSN: 2070-1721 G. Enyedi
Ericsson Ericsson
February 5, 2016 June 2016
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees An Architecture for IP/LDP Fast Reroute
draft-ietf-rtgwg-mrt-frr-architecture-10 Using Maximally Redundant Trees (MRT-FRR)
Abstract Abstract
This document defines the architecture for IP and LDP Fast-Reroute This document defines the architecture for IP and LDP Fast Reroute
using Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology using Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology
that gives link-protection and node-protection with 100% coverage in that gives link-protection and node-protection with 100% coverage in
any network topology that is still connected after the failure. any network topology that is still connected after the failure.
Status of This Memo Status of This Memo
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Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 4 1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 4
1.2. Partial Deployment and Backwards Compatibility . . . . . 5 1.2. Partial Deployment and Backwards Compatibility . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 7 4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 7
5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 9 5. MRT and Fast Reroute . . . . . . . . . . . . . . . . . . . . 9
6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 9 6. Unicast Forwarding with MRT Fast Reroute . . . . . . . . . . 9
6.1. Introduction to MRT Forwarding Options . . . . . . . . . 10 6.1. Introduction to MRT Forwarding Options . . . . . . . . . 10
6.1.1. MRT LDP labels . . . . . . . . . . . . . . . . . . . 10 6.1.1. MRT LDP Labels . . . . . . . . . . . . . . . . . . . 10
6.1.1.1. Topology-scoped FEC encoded using a single label 6.1.1.1. Topology-Scoped FEC Encoded Using a Single Label
(Option 1A) . . . . . . . . . . . . . . . . . . . 10 (Option 1A) . . . . . . . . . . . . . . . . . . . 10
6.1.1.2. Topology and FEC encoded using a two label stack 6.1.1.2. Topology and FEC Encoded Using a Two-Label Stack
(Option 1B) . . . . . . . . . . . . . . . . . . . 11 (Option 1B) . . . . . . . . . . . . . . . . . . . 11
6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B 12 6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B 12
6.1.1.4. Required support for MRT LDP Label options . . . 12 6.1.1.4. Required Support for MRT LDP Label Options . . . 12
6.1.2. MRT IP tunnels (Options 2A and 2B) . . . . . . . . . 12 6.1.2. MRT IP Tunnels (Options 2A and 2B) . . . . . . . . . 12
6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 13 6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 13
6.2.1. Forwarding LDP traffic using MRT LDP Label Option 1A 13 6.2.1. Forwarding LDP Traffic Using MRT LDP Label Option 1A 13
6.2.2. Forwarding LDP traffic using MRT LDP Label Option 1B 14 6.2.2. Forwarding LDP Traffic Using MRT LDP Label Option 1B 14
6.2.3. Other considerations for forwarding LDP traffic using 6.2.3. Other Considerations for Forwarding LDP Traffic Using
MRT LDP Labels . . . . . . . . . . . . . . . . . . . 14 MRT LDP Labels . . . . . . . . . . . . . . . . . . . 14
6.2.4. Required support for LDP traffic . . . . . . . . . . 14 6.2.4. Required Support for LDP Traffic . . . . . . . . . . 14
6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 14 6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 14
6.3.1. Tunneling IP traffic using MRT LDP Labels . . . . . . 15 6.3.1. Tunneling IP Traffic Using MRT LDP Labels . . . . . . 15
6.3.1.1. Tunneling IP traffic using MRT LDP Label Option 6.3.1.1. Tunneling IP Traffic Using MRT LDP Label Option
1A . . . . . . . . . . . . . . . . . . . . . . . 15 1A . . . . . . . . . . . . . . . . . . . . . . . 15
6.3.1.2. Tunneling IP traffic using MRT LDP Label Option 6.3.1.2. Tunneling IP Traffic Using MRT LDP Label Option
1B . . . . . . . . . . . . . . . . . . . . . . . 15 1B . . . . . . . . . . . . . . . . . . . . . . . 15
6.3.2. Tunneling IP traffic using MRT IP Tunnels . . . . . . 16 6.3.2. Tunneling IP Traffic Using MRT IP Tunnels . . . . . . 16
6.3.3. Required support for IP traffic . . . . . . . . . . . 16 6.3.3. Required Support for IP Traffic . . . . . . . . . . . 16
7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 16 7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 16
7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 17 7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 17
7.2. Support for a specific MRT profile . . . . . . . . . . . 17 7.2. Support for a Specific MRT Profile . . . . . . . . . . . 17
7.3. Excluding additional routers and interfaces from the MRT 7.3. Excluding Additional Routers and Interfaces from the MRT
Island . . . . . . . . . . . . . . . . . . . . . . . . . 18 Island . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3.1. Existing IGP exclusion mechanisms . . . . . . . . . . 18 7.3.1. Existing IGP Exclusion Mechanisms . . . . . . . . . . 18
7.3.2. MRT-specific exclusion mechanism . . . . . . . . . . 19 7.3.2. MRT-Specific Exclusion Mechanism . . . . . . . . . . 19
7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 19 7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 19
7.5. Algorithm for MRT Island Identification . . . . . . . . . 19 7.5. Algorithm for MRT Island Identification . . . . . . . . . 19
8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19 8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 19 8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 19
8.2. Router-specific MRT paramaters . . . . . . . . . . . . . 21 8.2. Router-Specific MRT Parameters . . . . . . . . . . . . . 21
8.3. Default MRT profile . . . . . . . . . . . . . . . . . . . 21 8.3. Default MRT Profile . . . . . . . . . . . . . . . . . . . 21
9. LDP signaling extensions and considerations . . . . . . . . . 22 9. LDP Signaling Extensions and Considerations . . . . . . . . . 22
10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 22 10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 23
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 23 10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 23
10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 24 10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 24
10.2. ABR Forwarding Behavior with IP Tunneling (option 2) . . 24 10.2. ABR Forwarding Behavior with IP Tunneling (Option 2) . . 24
10.3. ABR Forwarding Behavior with MRT LDP Label option 1B . . 25 10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B . . 25
11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 26 11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 26
11.1. Protecting Multi-Homed Prefixes using Tunnel Endpoint 11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint
Selection . . . . . . . . . . . . . . . . . . . . . . . 28 Selection . . . . . . . . . . . . . . . . . . . . . . . 28
11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 29 11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes . 29
11.3. MRT Alternates for Destinations Outside the MRT Island . 31 11.3. MRT Alternates for Destinations outside the MRT Island . 31
12. Network Convergence and Preparing for the Next Failure . . . 31 12. Network Convergence and Preparing for the Next Failure . . . 32
12.1. Micro-loop prevention and MRTs . . . . . . . . . . . . . 32 12.1. Micro-loop Prevention and MRTs . . . . . . . . . . . . . 32
12.2. MRT Recalculation for the Default MRT Profile . . . . . 33 12.2. MRT Recalculation for the Default MRT Profile . . . . . 33
13. Implementation Status . . . . . . . . . . . . . . . . . . . . 34 13. Operational Considerations . . . . . . . . . . . . . . . . . 34
14. Operational Considerations . . . . . . . . . . . . . . . . . 35 13.1. Verifying Forwarding on MRT Paths . . . . . . . . . . . 34
14.1. Verifying Forwarding on MRT Paths . . . . . . . . . . . 35 13.2. Traffic Capacity on Backup Paths . . . . . . . . . . . . 34
14.2. Traffic Capacity on Backup Paths . . . . . . . . . . . . 36 13.3. MRT IP Tunnel Loopback Address Management . . . . . . . 36
14.3. MRT IP Tunnel Loopback Address Management . . . . . . . 38 13.4. MRT-FRR in a Network with Degraded Connectivity . . . . 36
14.4. MRT-FRR in a Network with Degraded Connectivity . . . . 38 13.5. Partial Deployment of MRT-FRR in a Network . . . . . . . 37
14.5. Partial Deployment of MRT-FRR in a Network . . . . . . . 38 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39 15. Security Considerations . . . . . . . . . . . . . . . . . . . 38
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
17. Security Considerations . . . . . . . . . . . . . . . . . . . 40 16.1. Normative References . . . . . . . . . . . . . . . . . . 38
18. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 40 16.2. Informative References . . . . . . . . . . . . . . . . . 39
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 41 Appendix A. Inter-level Forwarding Behavior for IS-IS . . . . . 41
19.1. Normative References . . . . . . . . . . . . . . . . . . 41 Appendix B. General Issues with Area Abstraction . . . . . . . . 42
19.2. Informative References . . . . . . . . . . . . . . . . . 42 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 43
Appendix A. Inter-level Forwarding Behavior for IS-IS . . . . . 43 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Appendix B. General Issues with Area Abstraction . . . . . . . . 44 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction 1. Introduction
This document describes a solution for IP/LDP fast-reroute [RFC5714]. This document describes a solution for IP/LDP fast reroute [RFC5714].
MRT-FRR creates two alternate forwarding trees which are distinct MRT-FRR creates two alternate forwarding trees that are distinct from
from the primary next-hop forwarding used during stable operation. the primary next-hop forwarding used during stable operation. These
These two trees are maximally diverse from each other, providing link two trees are maximally diverse from each other, providing link and
and node protection for 100% of paths and failures as long as the node protection for 100% of paths and failures as long as the failure
failure does not cut the network into multiple pieces. This document does not cut the network into multiple pieces. This document defines
defines the architecture for IP/LDP fast-reroute with MRT. the architecture for IP/LDP fast reroute with MRT.
[I-D.ietf-rtgwg-mrt-frr-algorithm] describes how to compute maximally [RFC7811] describes how to compute maximally redundant trees using a
redundant trees using a specific algorithm, the MRT Lowpoint specific algorithm: the MRT Lowpoint algorithm. The MRT Lowpoint
algorithm. The MRT Lowpoint algorithm is used by a router that algorithm is used by a router that supports the Default MRT Profile,
supports the Default MRT Profile, as specified in this document. as specified in this document.
IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse IP/LDP Fast Reroute using Maximally Redundant Trees (MRT-FRR) uses
forwarding topologies to provide alternates. A primary next-hop two maximally diverse forwarding topologies to provide alternates. A
should be on only one of the diverse forwarding topologies; thus, the primary next hop should be on only one of the diverse forwarding
other can be used to provide an alternate. Once traffic has been topologies; thus, the other can be used to provide an alternate.
moved to one of the MRTs by one point of local repair (PLR), that Once traffic has been moved to one of the MRTs by one Point of Local
traffic is not subject to further repair actions by another PLR, even Repair (PLR), that traffic is not subject to further repair actions
in the event of multiple simultaneous failures. Therefore, traffic by another PLR, even in the event of multiple simultaneous failures.
repaired by MRT-FRR will not loop between different PLRs responding Therefore, traffic repaired by MRT-FRR will not loop between
to different simultaneous failures. different PLRs responding to different simultaneous failures.
While MRT provides 100% protection for a single link or node failure, While MRT provides 100% protection for a single link or node failure,
it may not protect traffic in the event of multiple simultaneous it may not protect traffic in the event of multiple simultaneous
failures, nor does take into account Shared Risk Link Groups (SRLGs). failures, nor does it take into account Shared Risk Link Groups
Also, while the MRT Lowpoint algorithm is computationally efficient, (SRLGs). Also, while the MRT Lowpoint algorithm is computationally
it is also new. In order for MRT-FRR to function properly, all of efficient, it is also new. In order for MRT-FRR to function
the other nodes in the network that support MRT must correctly properly, all of the other nodes in the network that support MRT must
compute next-hops based on the same algorithm, and install the correctly compute next hops based on the same algorithm and install
corresponding forwarding state. This is in contrast to other FRR the corresponding forwarding state. This is in contrast to other FRR
methods where the calculation of backup paths generally involves methods where the calculation of backup paths generally involves
repeated application of the simpler and widely-deployed shortest path repeated application of the simpler and widely deployed Shortest Path
first (SPF) algorithm, and backup paths themselves re-use the First (SPF) algorithm, and backup paths themselves reuse the
forwarding state used for shortest path forwarding of normal traffic. forwarding state used for shortest path forwarding of normal traffic.
Section 14 provides operational guidance related to verification of Section 13 provides operational guidance related to verification of
MRT forwarding paths. MRT forwarding paths.
In addition to supporting IP and LDP unicast fast-reroute, the In addition to supporting IP and LDP unicast fast reroute, the
diverse forwarding topologies and guarantee of 100% coverage permit diverse forwarding topologies and guarantee of 100% coverage permit
fast-reroute technology to be applied to multicast traffic as fast-reroute technology to be applied to multicast traffic as
described in [I-D.atlas-rtgwg-mrt-mc-arch]. However, the current described in [MRT-ARCH]. However, the current document does not
document does not address the multicast applications of MRTs. address the multicast applications of MRTs.
1.1. Importance of 100% Coverage 1.1. Importance of 100% Coverage
Fast-reroute is based upon the single failure assumption - that the Fast reroute is based upon the single failure assumption: that the
time between single failures is long enough for a network to time between single failures is long enough for a network to
reconverge and start forwarding on the new shortest paths. That does reconverge and start forwarding on the new shortest paths. That does
not imply that the network will only experience one failure or not imply that the network will only experience one failure or
change. change.
It is straightforward to analyze a particular network topology for It is straightforward to analyze a particular network topology for
coverage. However, a real network does not always have the same coverage. However, a real network does not always have the same
topology. For instance, maintenance events will take links or nodes topology. For instance, maintenance events will take links or nodes
out of use. Simply costing out a link can have a significant effect out of use. Simply costing out a link can have a significant effect
on what loop-free alternates (LFAs) are available. Similarly, after on what Loop-Free Alternates (LFAs) are available. Similarly, after
a single failure has happened, the topology is changed and its a single failure has happened, the topology is changed and its
associated coverage. Finally, many networks have new routers or associated coverage has changed as well. Finally, many networks have
links added and removed; each of those changes can have an effect on new routers or links added and removed; each of those changes can
the coverage for topology-sensitive methods such as LFA and Remote have an effect on the coverage for topology-sensitive methods such as
LFA. If fast-reroute is important for the network services provided, LFA and Remote LFA. If fast reroute is important for the network
then a method that guarantees 100% coverage is important to services provided, then a method that guarantees 100% coverage is
accommodate natural network topology changes. important to accommodate natural network topology changes.
When a network needs to use Ordered FIB[RFC6976] or Nearside When a network needs to use Ordered FIB [RFC6976] or Nearside
Tunneling[RFC5715] as a micro-loop prevention mechanism [RFC5715], Tunneling [RFC5715] as a micro-loop prevention mechanism [RFC5715],
then the whole IGP area needs to have alternates available. This then the whole IGP area needs to have alternates available. This
allows the micro-loop prevention mechanism, which requires slower allows the micro-loop prevention mechanism, which requires slower
network convergence, to take the necessary time without adversely network convergence, to take the necessary time without adversely
impacting traffic. Without complete coverage, traffic to the impacting traffic. Without complete coverage, traffic to the
unprotected destinations will be dropped for significantly longer unprotected destinations will be dropped for significantly longer
than with current convergence - where routers individually converge than with current convergence -- where routers individually converge
as fast as possible. See Section 12.1 for more discussion of micro- as fast as possible. See Section 12.1 for more discussion of micro-
loop prevention and MRTs. loop prevention and MRTs.
1.2. Partial Deployment and Backwards Compatibility 1.2. Partial Deployment and Backwards Compatibility
MRT-FRR supports partial deployment. Routers advertise their ability MRT-FRR supports partial deployment. Routers advertise their ability
to support MRT. Inside the MRT-capable connected group of routers to support MRT. Inside the MRT-capable connected group of routers
(referred to as an MRT Island), the MRTs are computed. Alternates to (referred to as an MRT Island), the MRTs are computed. Alternates to
destinations outside the MRT Island are computed and depend upon the destinations outside the MRT Island are computed and depend upon the
existence of a loop-free neighbor of the MRT Island for that existence of a loop-free neighbor of the MRT Island for that
destination. MRT Islands are discussed in detail in Section 7, and destination. MRT Islands are discussed in detail in Section 7, and
partial deployment is discussed in more detail in Section 14.5. partial deployment is discussed in more detail in Section 13.5.
2. Requirements Language 2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Terminology 3. Terminology
network graph: A graph that reflects the network topology where all network graph: A graph that reflects the network topology where all
links connect exactly two nodes and broadcast links have been links connect exactly two nodes and broadcast links have been
transformed into the standard pseudo-node representation. transformed into the standard pseudonode representation.
cut-link: A link whose removal partitions the network. A cut-link cut-link: A link whose removal partitions the network. A cut-link
by definition must be connected between two cut-vertices. If by definition must be connected between two cut-vertices. If
there are multiple parallel links, then they are referred to as there are multiple parallel links, then they are referred to as
cut-links in this document if removing the set of parallel links cut-links in this document if removing the set of parallel links
would partition the network graph. would partition the network graph.
cut-vertex: A vertex whose removal partitions the network graph. cut-vertex: A vertex whose removal partitions the network graph.
2-connected: A graph that has no cut-vertices. This is a graph 2-connected: A graph that has no cut-vertices. This is a graph
that requires two nodes to be removed before the network is that requires two nodes to be removed before the network is
partitioned. partitioned.
2-connected cluster: A maximal set of nodes that are 2-connected. 2-connected cluster: A maximal set of nodes that are 2-connected.
block: Either a 2-connected cluster, a cut-edge, or an isolated block: Either a 2-connected cluster, a cut-edge, or a cut-vertex.
vertex.
Redundant Trees (RT): A pair of trees where the path from any node Redundant Trees (RT): A pair of trees where the path from any node
X to the root R along the first tree is node-disjoint with the X to the root R along the first tree is node-disjoint with the
path from the same node X to the root along the second tree. path from the same node X to the root along the second tree.
Redundant trees can always be computed in 2-connected graphs. Redundant trees can always be computed in 2-connected graphs.
Maximally Redundant Trees (MRT): A pair of trees where the path Maximally Redundant Trees (MRT): A pair of trees where the path
from any node X to the root R along the first tree and the path from any node X to the root R along the first tree and the path
from the same node X to the root along the second tree share the from the same node X to the root along the second tree share the
minimum number of nodes and the minimum number of links. Each minimum number of nodes and the minimum number of links. Each
skipping to change at page 6, line 30 skipping to change at page 6, line 25
In graphs that are not 2-connected, it is not possible to compute In graphs that are not 2-connected, it is not possible to compute
RTs. However, it is possible to compute MRTs. MRTs are maximally RTs. However, it is possible to compute MRTs. MRTs are maximally
redundant in the sense that they are as redundant as possible redundant in the sense that they are as redundant as possible
given the constraints of the network graph. given the constraints of the network graph.
Directed Acyclic Graph (DAG): A graph where all links are directed Directed Acyclic Graph (DAG): A graph where all links are directed
and there are no cycles in it. and there are no cycles in it.
Almost Directed Acyclic Graph (ADAG): A graph with one node Almost Directed Acyclic Graph (ADAG): A graph with one node
designated as the root. The graph has the property that if all designated as the root. The graph has the property that if all
links incoming to the root were removed, then resulting graph links incoming to the root were removed, then the resulting graph
would be a DAG. would be a DAG.
Generalized ADAG (GADAG): A graph that is the combination of the Generalized ADAG (GADAG): A graph that is the combination of the
ADAGs of all blocks. ADAGs of all blocks.
MRT-Red: MRT-Red is used to describe one of the two MRTs; it is MRT-Red: MRT-Red is used to describe one of the two MRTs; it is
used to describe the associated forwarding topology and MPLS used to describe the associated forwarding topology and MPLS
multi-topology identifier (MT-ID). Specifically, MRT-Red is the Multi-Topology IDentifier (MT-ID). Specifically, MRT-Red is the
decreasing MRT where links in the GADAG are taken in the direction decreasing MRT where links in the GADAG are taken in the direction
from a higher topologically ordered node to a lower one. from a higher topologically ordered node to a lower one.
MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MPLS MT- used to described the associated forwarding topology and MPLS
ID. Specifically, MRT-Blue is the increasing MRT where links in MT-ID. Specifically, MRT-Blue is the increasing MRT where links
the GADAG are taken in the direction from a lower topologically in the GADAG are taken in the direction from a lower topologically
ordered node to a higher one. ordered node to a higher one.
Rainbow MRT: It is useful to have an MPLS MT-ID that refers to the Rainbow MRT: It is useful to have an MPLS MT-ID that refers to the
multiple MRT forwarding topologies and to the default forwarding multiple MRT forwarding topologies and to the default forwarding
topology. This is referred to as the Rainbow MRT MPLS MT-ID and topology. This is referred to as the Rainbow MRT MPLS MT-ID and
is used by LDP to reduce signaling and permit the same label to is used by LDP to reduce signaling and permit the same label to
always be advertised to all peers for the same (MT-ID, Prefix). always be advertised to all peers for the same (MT-ID, Prefix).
MRT Island: The set of routers that support a particular MRT MRT Island: The set of routers that support a particular MRT
profile and the links connecting them that support MRT. profile and the links connecting them that support MRT.
Island Border Router (IBR): A router in the MRT Island that is Island Border Router (IBR): A router in the MRT Island that is
connected to a router not in the MRT Island and both routers are connected to a router not in the MRT Island, both of which are in
in a common area or level. a common area or level.
Island Neighbor (IN): A router that is not in the MRT Island but is Island Neighbor (IN): A router that is not in the MRT Island but is
adjacent to an IBR and in the same area/level as the IBR. adjacent to an IBR and in the same area/level as the IBR.
named proxy-node: A proxy-node can represent a destination prefix named proxy-node: A proxy-node can represent a destination prefix
that can be attached to the MRT Island via at least two routers. that can be attached to the MRT Island via at least two routers.
It is named if there is a way that traffic can be encapsulated to It is named if there is a way that traffic can be encapsulated to
reach specifically that proxy node; this could be because there is reach specifically that proxy node; this could be because there is
an LDP FEC (Forwarding Equivalence Class) for the associated an LDP FEC (Forwarding Equivalence Class) for the associated
prefix or because MRT-Red and MRT-Blue IP addresses are advertised prefix or because MRT-Red and MRT-Blue IP addresses are advertised
in an undefined fashion for that proxy-node. in an undefined fashion for that proxy-node.
4. Maximally Redundant Trees (MRT) 4. Maximally Redundant Trees (MRT)
A pair of Maximally Redundant Trees is a pair of directed spanning A pair of Maximally Redundant Trees is a pair of directed spanning
trees that provides maximally disjoint paths towards their common trees that provides maximally disjoint paths towards their common
root. Only links or nodes whose failure would partition the network root. Only links or nodes whose failure would partition the network
(i.e. cut-links and cut-vertices) are shared between the trees. The (i.e., cut-links and cut-vertices) are shared between the trees. The
MRT Lowpoint algorithm is given in MRT Lowpoint algorithm is given in [RFC7811]. This algorithm can be
[I-D.ietf-rtgwg-mrt-frr-algorithm]. This algorithm can be computed computed in O(e + n log n); it is less than three SPFs. This
in O(e + n log n); it is less than three SPFs. This document document describes how the MRTs can be used and not how to compute
describes how the MRTs can be used and not how to compute them. them.
MRT provides destination-based trees for each destination. Each MRT provides destination-based trees for each destination. Each
router stores its normal primary next-hop(s) as well as MRT-Blue router stores its normal primary next hop(s) as well as MRT-Blue next
next-hop(s) and MRT-Red next-hop(s) toward each destination. The hop(s) and MRT-Red next hop(s) toward each destination. The
alternate will be selected between the MRT-Blue and MRT-Red. alternate will be selected between the MRT-Blue and MRT-Red.
The most important thing to understand about MRTs is that for each The most important thing to understand about MRTs is that for each
pair of destination-routed MRTs, there is a path from every node X to pair of destination-routed MRTs, there is a path from every node X to
the destination D on the Blue MRT that is as disjoint as possible the destination D on the Blue MRT that is as disjoint as possible
from the path on the Red MRT. from the path on the Red MRT.
For example, in Figure 1, there is a network graph that is For example, in Figure 1, there is a network graph that is
2-connected in (a) and associated MRTs in (b) and (c). One can 2-connected in (a) and associated MRTs in (b) and (c). One can
consider the paths from B to R; on the Blue MRT, the paths are consider the paths from B to R; on the Blue MRT, the paths are
skipping to change at page 8, line 16 skipping to change at page 8, line 16
| | | | ^ | | | | | | | ^ | | |
| | | V | | V V | | | V | | V V
[R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C]
| | | ^ ^ ^ | | | | | ^ ^ ^ | |
| | | | | | V | | | | | | | V |
[A]---[B]---| [A]-->[B]---| [A]<--[B]<--| [A]---[B]---| [A]-->[B]---| [A]<--[B]<--|
(a) (b) (c) (a) (b) (c)
a 2-connected graph Blue MRT towards R Red MRT towards R a 2-connected graph Blue MRT towards R Red MRT towards R
Figure 1: A 2-connected Network Figure 1: A 2-Connected Network
By contrast, in Figure 2, the network in (a) is not 2-connected. If By contrast, in Figure 2, the network in (a) is not 2-connected. If
F, G or the link F<->G failed, then the network would be partitioned. C, G, or the link C<->G failed, then the network would be
It is clearly impossible to have two link-disjoint or node-disjoint partitioned. It is clearly impossible to have two link-disjoint or
paths from G, I or J to R. The MRTs given in (b) and (c) offer paths node-disjoint paths from G, J, or H to R. The MRTs given in (b) and
that are as disjoint as possible. For instance, the paths from B to (c) offer paths that are as disjoint as possible. For instance, the
R are the same as in Figure 1 and the path from G to R on the Blue paths from B to R are the same as in Figure 1 and the path from G to
MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R. R on the Blue MRT is G->C->D->E->R and on the Red MRT is
G->C->B->A->R.
[E]---[D]---| [E]---[D]---| |---[J]
| | | |----[I] | | | | |
| | | | | | | | | |
[R]---[C] [F]---[G] | [R] [F] [C]---[G] |
| | | | | | | | | |
| | | |----[J] | | | | |
[A]---[B]---| [A]---[B]---| |---[H]
(a) (a) a graph that is not 2-connected
a non-2-connected graph
[E]<--[D]<--| [E]-->[D] [E]<--[D]<--| [J] [E]-->[D]---| |---[J]
| ^ | [I] | |----[I] | ^ | | | | | ^
V | | | V V ^ V | | | V V V |
[R] [C] [F]<--[G] | [R]<--[C] [F]<--[G] | [R] [F] [C]<--[G] | [R] [F] [C]<--[G] |
^ ^ ^ V ^ | | ^ ^ ^ | ^ | | |
| | |----[J] | | [J] | | | V | V | |
[A]-->[B]---| [A]<--[B]<--| [A]-->[B]---| |---[H] [A]<--[B]<--| [H]
(b) (c) (b) Blue MRT towards R (c) Red MRT towards R
Blue MRT towards R Red MRT towards R
Figure 2: A non-2-connected network Figure 2: A Network That Is Not 2-Connected
5. Maximally Redundant Trees (MRT) and Fast-Reroute 5. MRT and Fast Reroute
In normal IGP routing, each router has its shortest path tree (SPT) In normal IGP routing, each router has its Shortest Path Tree (SPT)
to all destinations. From the perspective of a particular to all destinations. From the perspective of a particular
destination, D, this looks like a reverse SPT. To use maximally destination, D, this looks like a reverse SPT (rSPT). To use MRT, in
redundant trees, in addition, each destination D has two MRTs addition, each destination D has two MRTs associated with it; by
associated with it; by convention these will be called the MRT-Blue convention these will be called the MRT-Blue and MRT-Red. MRT-FRR is
and MRT-Red. MRT-FRR is realized by using multi-topology forwarding. realized by using multi-topology forwarding. There is a MRT-Blue
There is a MRT-Blue forwarding topology and a MRT-Red forwarding forwarding topology and a MRT-Red forwarding topology.
topology.
Any IP/LDP fast-reroute technique beyond LFA requires an additional Any IP/LDP fast-reroute technique beyond LFA requires an additional
dataplane procedure, such as an additional forwarding mechanism. The dataplane procedure, such as an additional forwarding mechanism. The
well-known options are multi-topology forwarding (used by MRT-FRR), well-known options are multi-topology forwarding (used by MRT-FRR),
tunneling (e.g. [RFC6981] or [RFC7490]), and per-interface tunneling (e.g., [RFC6981] or [RFC7490]), and per-interface
forwarding (e.g. Loop-Free Failure Insensitive Routing in forwarding (e.g., Loop-Free Failure Insensitive Routing in
[EnyediThesis]). [EnyediThesis]).
When there is a link or node failure affecting, but not partitioning, When there is a link or node failure affecting, but not partitioning,
the network, each node will still have at least one path via one of the network, each node will still have at least one path via one of
the MRTs to reach the destination D. For example, in Figure 2, C the MRTs to reach the destination D. For example, in Figure 2, B
would normally forward traffic to R across the C<->R link. If that would normally forward traffic to R across the path B->A->R. If the
C<->R link fails, then C could use the Blue MRT path C->D->E->R. B<->A link fails, then B could use the MRT-Blue path B->F->D->E->R.
As is always the case with fast-reroute technologies, forwarding does As is always the case with fast-reroute technologies, forwarding does
not change until a local failure is detected. Packets are forwarded not change until a local failure is detected. Packets are forwarded
along the shortest path. The appropriate alternate to use is pre- along the shortest path. The appropriate alternate to use is pre-
computed. [I-D.ietf-rtgwg-mrt-frr-algorithm] describes exactly how computed. [RFC7811] describes exactly how to determine whether the
to determine whether the MRT-Blue next-hops or the MRT-Red next-hops MRT-Blue next hops or the MRT-Red next hops should be the MRT
should be the MRT alternate next-hops for a particular primary next- alternate next hops for a particular primary next hop to a particular
hop to a particular destination. destination.
MRT alternates are always available to use. It is a local decision MRT alternates are always available to use. It is a local decision
whether to use an MRT alternate, a Loop-Free Alternate or some other whether to use an MRT alternate, an LFA, or some other type of
type of alternate. alternate.
As described in [RFC5286], when a worse failure than is anticipated As described in [RFC5286], when a worse failure than is anticipated
happens, using LFAs that are not downstream neighbors can cause happens, using LFAs that are not downstream neighbors can cause
looping among alternates. Section 1.1 of [RFC5286] gives an example looping among alternates. Section 1.1 of [RFC5286] gives an example
of link-protecting alternates causing a loop on node failure. Even of link-protecting alternates causing a loop on node failure. Even
if a worse failure than anticipated happens, the use of MRT if a worse failure than anticipated happens, the use of MRT
alternates will not cause looping. alternates will not cause looping.
6. Unicast Forwarding with MRT Fast-Reroute 6. Unicast Forwarding with MRT Fast Reroute
There are three possible types of routers involved in forwarding a There are three possible types of routers involved in forwarding a
packet along an MRT path. At the MRT ingress router, the packet packet along an MRT path. At the MRT ingress router, the packet
leaves the shortest path to the destination and follows an MRT path leaves the shortest path to the destination and follows an MRT path
to the destination. In an FRR application, the MRT ingress router is to the destination. In an FRR application, the MRT ingress router is
the PLR. An MRT transit router takes a packet that arrives already the PLR. An MRT transit router takes a packet that arrives already
associated with the particular MRT, and forwards it on that same MRT. associated with the particular MRT, and forwards it on that same MRT.
In some situations (to be discussed later), the packet will need to In some situations (to be discussed later), the packet will need to
leave the MRT path and return to the shortest path. This takes place leave the MRT path and return to the shortest path. This takes place
at the MRT egress router. The MRT ingress and egress functionality at the MRT egress router. The MRT ingress and egress functionality
may depend on the underlying type of packet being forwarded (LDP or may depend on the underlying type of packet being forwarded (LDP or
IP). The MRT transit functionality is independent of the type of IP). The MRT transit functionality is independent of the type of
packet being forwarded. We first consider several MRT transit packet being forwarded. We first consider several MRT transit
forwarding mechanisms. Then we look at how these forwarding forwarding mechanisms. Then, we look at how these forwarding
mechanisms can be applied to carrying LDP and IP traffic. mechanisms can be applied to carrying LDP and IP traffic.
6.1. Introduction to MRT Forwarding Options 6.1. Introduction to MRT Forwarding Options
The following options for MRT forwarding mechanisms are considered. The following options for MRT forwarding mechanisms are considered.
1. MRT LDP Labels 1. MRT LDP Labels
A. Topology-scoped FEC encoded using a single label A. Topology-scoped FEC encoded using a single label
B. Topology and FEC encoded using a two label stack B. Topology and FEC encoded using a two-label stack
2. MRT IP Tunnels 2. MRT IP Tunnels
A. MRT IPv4 Tunnels A. MRT IPv4 Tunnels
B. MRT IPv6 Tunnels B. MRT IPv6 Tunnels
6.1.1. MRT LDP labels 6.1.1. MRT LDP Labels
We consider two options for the MRT forwarding mechanisms using MRT We consider two options for the MRT forwarding mechanisms using MRT
LDP labels. LDP labels.
6.1.1.1. Topology-scoped FEC encoded using a single label (Option 1A) 6.1.1.1. Topology-Scoped FEC Encoded Using a Single Label (Option 1A)
[RFC7307] provides a mechanism to distribute FEC-Label bindings [RFC7307] provides a mechanism to distribute FEC-label bindings
scoped to a given MPLS topology (represented by MPLS MT-ID). To use scoped to a given MPLS topology (represented by MPLS MT-ID). To use
multi-topology LDP to create MRT forwarding topologies, we associate multi-topology LDP to create MRT forwarding topologies, we associate
two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies, two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies,
in addition to the default shortest path forwarding topology with MT- in addition to the default shortest path forwarding topology with
ID=0. MT-ID=0.
With this forwarding mechanism, a single label is distributed for With this forwarding mechanism, a single label is distributed for
each topology-scoped FEC. For a given FEC in the default topology each topology-scoped FEC. For a given FEC in the default topology
(call it default-FEC-A), two additional topology-scoped FECs would be (call it default-FEC-A), two additional topology-scoped FECs would be
created, corresponding to the Red and Blue MRT forwarding topologies created, corresponding to the Red and Blue MRT forwarding topologies
(call them red-FEC-A and blue-FEC-A). A router supporting this MRT (call them red-FEC-A and blue-FEC-A). A router supporting this MRT
transit forwarding mechanism advertises a different FEC-label binding transit forwarding mechanism advertises a different FEC-label binding
for each of the three topology-scoped FECs. When a packet is for each of the three topology-scoped FECs. When a packet is
received with a label corresponding to red-FEC-A (for example), an received with a label corresponding to red-FEC-A (for example), an
MRT transit router will determine the next-hop for the MRT-Red MRT transit router will determine the next hop for the MRT-Red
forwarding topology for that FEC, swap the incoming label with the forwarding topology for that FEC, swap the incoming label with the
outgoing label corresponding to red-FEC-A learned from the MRT-Red outgoing label corresponding to red-FEC-A learned from the MRT-Red
next-hop router, and forward the packet. next-hop router, and forward the packet.
This forwarding mechanism has the useful property that the FEC This forwarding mechanism has the useful property that the FEC
associated with the packet is maintained in the labels at each hop associated with the packet is maintained in the labels at each hop
along the MRT. We will take advantage of this property when along the MRT. We will take advantage of this property when
specifying how to carry LDP traffic on MRT paths using multi-topology specifying how to carry LDP traffic on MRT paths using multi-topology
LDP labels. LDP labels.
This approach is very simple for hardware to support. However, it This approach is very simple for hardware to support. However, it
reduces the label space for other uses, and it increases the memory reduces the label space for other uses, and it increases the memory
needed to store the labels and the communication required by LDP to needed to store the labels and the communication required by LDP to
distribute FEC-label bindings. In general, this approach will also distribute FEC-label bindings. In general, this approach will also
increase the time needed to install the FRR entries in the Forwarding increase the time needed to install the FRR entries in the Forwarding
Information Base (FIB) and hence the time needed before the next Information Base (FIB) and, hence, the time needed before the next
failure can be protected. failure can be protected.
This forwarding option uses the LDP signaling extensions described in This forwarding option uses the LDP signaling extensions described in
[RFC7307]. The MRT-specific LDP extensions required to support this [RFC7307]. The MRT-specific LDP extensions required to support this
option will be described elsewhere. option will be described elsewhere.
6.1.1.2. Topology and FEC encoded using a two label stack (Option 1B) 6.1.1.2. Topology and FEC Encoded Using a Two-Label Stack (Option 1B)
With this forwarding mechanism, a two label stack is used to encode With this forwarding mechanism, a two-label stack is used to encode
the topology and the FEC of the packet. The top label (topology-id the topology and the FEC of the packet. The top label (topology-id
label) identifies the MRT forwarding topology, while the second label label) identifies the MRT forwarding topology, while the second label
(FEC label) identifies the FEC. The top label would be a new FEC (FEC label) identifies the FEC. The top label would be a new FEC
type with two values corresponding to MRT Red and Blue topologies. type with two values corresponding to MRT Red and Blue topologies.
When an MRT transit router receives a packet with a topology-id When an MRT transit router receives a packet with a topology-id
label, the router pops the top label and uses that it to guide the label, the router pops the top label and uses that it to guide the
next-hop selection in combination with the next label in the stack next-hop selection in combination with the next label in the stack
(the FEC label). The router then swaps the FEC label, using the FEC- (the FEC label). The router then swaps the FEC label, using the FEC-
label bindings learned through normal LDP mechanisms. The router label bindings learned through normal LDP mechanisms. The router
then pushes the topology-id label for the next-hop. then pushes the topology-id label for the next hop.
As with Option 1A, this forwarding mechanism also has the useful As with Option 1A, this forwarding mechanism also has the useful
property that the FEC associated with the packet is maintained in the property that the FEC associated with the packet is maintained in the
labels at each hop along the MRT. labels at each hop along the MRT.
This forwarding mechanism has minimal usage of additional labels, This forwarding mechanism has minimal usage of additional labels,
memory and LDP communication. It does increase the size of packets memory and LDP communication. It does increase the size of packets
and the complexity of the required label operations and look-ups. and the complexity of the required label operations and lookups.
This forwarding option is consistent with context-specific label This forwarding option is consistent with context-specific label
spaces, as described in [RFC5331]. However, the precise LDP behavior spaces, as described in [RFC5331]. However, the precise LDP behavior
required to support this option for MRT has not been specified. required to support this option for MRT has not been specified.
6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B 6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B
MRT transit forwarding based on MRT LDP Label options 1A and 1B can MRT transit forwarding based on MRT LDP Label options 1A and 1B can
coexist in the same network, with a packet being forwarded along a coexist in the same network, with a packet being forwarded along a
single MRT path using the single label of option 1A for some hops and single MRT path using the single label of Option 1A for some hops and
the two label stack of option 1B for other hops. However, to the two-label stack of Option 1B for other hops. However, to
simplify the process of MRT Island formation we require that all simplify the process of MRT Island formation, we require that all
routers in the MRT Island support at least one common forwarding routers in the MRT Island support at least one common forwarding
mechanism. As an example, the Default MRT Profile requires support mechanism. As an example, the Default MRT Profile requires support
for the MRT LDP Label Option 1A forwarding mechanism. This ensures for the MRT LDP Label Option 1A forwarding mechanism. This ensures
that the routers in an MRT island supporting the Default MRT Profile that the routers in an MRT island supporting the Default MRT Profile
will be able to establish MRT forwarding paths based on MRT LDP Label will be able to establish MRT forwarding paths based on MRT LDP Label
Option 1A. However, an implementation supporting Option 1A may also Option 1A. However, an implementation supporting Option 1A may also
support Option 1B. If the scaling or performance characteristics for support Option 1B. If the scaling or performance characteristics for
the two options differ in this implementation, then it may be the two options differ in this implementation, then it may be
desirable for a pair of adjacent routers to use Option 1B labels desirable for a pair of adjacent routers to use Option 1B labels
instead of the Option 1A labels. If those routers successfully instead of the Option 1A labels. If those routers successfully
negotiate the use of Option 1B labels, they are free to use them. negotiate the use of Option 1B labels, they are free to use them.
This can occur without any of the other routers in the MRT Island This can occur without any of the other routers in the MRT Island
being made aware of it. being made aware of it.
Note that this document only defines the Default MRT Profile which Note that this document only defines the Default MRT Profile, which
requires support for the MRT LDP Label Option 1A forwarding requires support for the MRT LDP Label Option 1A forwarding
mechanism. mechanism.
6.1.1.4. Required support for MRT LDP Label options 6.1.1.4. Required Support for MRT LDP Label Options
If a router supports a profile that includes the MRT LDP Label Option If a router supports a profile that includes the MRT LDP Label Option
1A for the MRT transit forwarding mechanism, then it MUST support 1A for the MRT transit forwarding mechanism, then it MUST support
option 1A, which encodes topology-scoped FECs using a single label. Option 1A, which encodes topology-scoped FECs using a single label.
The router MAY also support option 1B. The router MAY also support Option 1B.
If a router supports a profile that includes the MRT LDP Label Option If a router supports a profile that includes the MRT LDP Label Option
1B for the MRT transit forwarding mechanism, then it MUST support 1B for the MRT transit forwarding mechanism, then it MUST support
option 1B, which encodes the topology and FEC using a two label Option 1B, which encodes the topology and FEC using a two-label
stack. The router MAY also support option 1A. stack. The router MAY also support Option 1A.
6.1.2. MRT IP tunnels (Options 2A and 2B) 6.1.2. MRT IP Tunnels (Options 2A and 2B)
IP tunneling can also be used as an MRT transit forwarding mechanism. IP tunneling can also be used as an MRT transit forwarding mechanism.
Each router supporting this MRT transit forwarding mechanism Each router supporting this MRT transit forwarding mechanism
announces two additional loopback addresses and their associated MRT announces two additional loopback addresses and their associated MRT
color. Those addresses are used as destination addresses for MRT- color. Those addresses are used as destination addresses for MRT-
blue and MRT-red IP tunnels respectively. The special loopback blue and MRT-red IP tunnels, respectively. The special loopback
addresses allow the transit nodes to identify the traffic as being addresses allow the transit nodes to identify the traffic as being
forwarded along either the MRT-blue or MRT-red topology to reach the forwarded along either the MRT-blue or MRT-red topology to reach the
tunnel destination. For example, an MRT ingress router can cause a tunnel destination. For example, an MRT ingress router can cause a
packet to be tunneled along the MRT-red path to router X by packet to be tunneled along the MRT-red path to router X by
encapsulating the packet using the MRT-red loopback address encapsulating the packet using the MRT-red loopback address
advertised by router X. Upon receiving the packet, router X would advertised by router X. Upon receiving the packet, router X would
remove the encapsulation header and forward the packet based on the remove the encapsulation header and forward the packet based on the
original destination address. original destination address.
Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the Either IPv4 (Option 2A) or IPv6 (Option 2B) can be used as the
tunneling mechanism. tunneling mechanism.
Note that the two forwarding mechanisms using LDP Label options do Note that the two forwarding mechanisms using LDP Label options do
not require additional loopbacks per router, as is required by the IP not require additional loopbacks per router, as is required by the IP
tunneling mechanism. This is because LDP labels are used on a hop- tunneling mechanism. This is because LDP labels are used on a hop-
by-hop basis to identify MRT-blue and MRT-red forwarding topologies. by-hop basis to identify MRT-blue and MRT-red forwarding topologies.
6.2. Forwarding LDP Unicast Traffic over MRT Paths 6.2. Forwarding LDP Unicast Traffic over MRT Paths
In the previous section, we examined several options for providing In the previous section, we examined several options for providing
skipping to change at page 13, line 39 skipping to change at page 13, line 39
We also simplify the initial discussion by assuming that the network We also simplify the initial discussion by assuming that the network
consists of a single IGP area, and that all routers in the network consists of a single IGP area, and that all routers in the network
participate in MRT. Other deployment scenarios that require MRT participate in MRT. Other deployment scenarios that require MRT
egress functionality are considered later in this document. egress functionality are considered later in this document.
In principle, it is possible to carry LDP traffic in MRT IP tunnels. In principle, it is possible to carry LDP traffic in MRT IP tunnels.
However, for LDP traffic, it is desirable to avoid tunneling. However, for LDP traffic, it is desirable to avoid tunneling.
Tunneling LDP traffic to a remote node requires knowledge of remote Tunneling LDP traffic to a remote node requires knowledge of remote
FEC-label bindings so that the LDP traffic can continue to be FEC-label bindings so that the LDP traffic can continue to be
forwarded properly when it leaves the tunnel. This requires targeted forwarded properly when it leaves the tunnel. This requires targeted
LDP sessions which can add management complexity. As described LDP sessions, which can add management complexity. As described
below, the two MRT forwarding mechanisms that use LDP labels do not below, the two MRT forwarding mechanisms that use LDP labels do not
require targeted LDP sessions. require targeted LDP sessions.
6.2.1. Forwarding LDP traffic using MRT LDP Label Option 1A 6.2.1. Forwarding LDP Traffic Using MRT LDP Label Option 1A
The MRT LDP Label option 1A forwarding mechanism uses topology-scoped The MRT LDP Label Option 1A forwarding mechanism uses topology-scoped
FECs encoded using a single label as described in section FECs encoded using a single label as described in Section 6.1.1.1.
Section 6.1.1.1. When a PLR receives an LDP packet that needs to be When a PLR receives an LDP packet that needs to be forwarded on the
forwarded on the Red MRT (for example), it does a label swap MRT-Red (for example), it does a label swap operation, replacing the
operation, replacing the usual LDP label for the FEC with the Red MRT usual LDP label for the FEC with the MRT-Red label for that FEC
label for that FEC received from the next-hop router in the Red MRT received from the next-hop router in the MRT-Red computed by the PLR.
computed by the PLR. When the next-hop router in the Red MRT When the next-hop router in the MRT-Red receives the packet with the
receives the packet with the Red MRT label for the FEC, the MRT MRT-Red label for the FEC, the MRT transit forwarding functionality
transit forwarding functionality continues as described in continues as described in Section 6.1.1.1. In this way, the original
Section 6.1.1.1. In this way the original FEC associated with the FEC associated with the packet is maintained at each hop along the
packet is maintained at each hop along the MRT. MRT.
6.2.2. Forwarding LDP traffic using MRT LDP Label Option 1B 6.2.2. Forwarding LDP Traffic Using MRT LDP Label Option 1B
The MRT LDP Label option 1B forwarding mechanism encodes the topology The MRT LDP Label Option 1B forwarding mechanism encodes the topology
and the FEC using a two label stack as described in Section 6.1.1.2. and the FEC using a two-label stack as described in Section 6.1.1.2.
When a PLR receives an LDP packet that needs to be forwarded on the When a PLR receives an LDP packet that needs to be forwarded on the
Red MRT, it first does a normal LDP label swap operation, replacing MRT-Red, it first does a normal LDP label swap operation, replacing
the incoming normal LDP label associated with a given FEC with the the incoming normal LDP label associated with a given FEC with the
outgoing normal LDP label for that FEC learned from the next-hop on outgoing normal LDP label for that FEC learned from the next hop on
the Red MRT. In addition, the PLR pushes the topology-identification the MRT-Red. In addition, the PLR pushes the topology-id label
label associated with the Red MRT, and forward the packet to the associated with the MRT-Red, and forward the packet to the
appropriate next-hop on the Red MRT. When the next-hop router in the appropriate next hop on the MRT-Red. When the next-hop router in the
Red MRT receives the packet with the Red MRT label for the FEC, the MRT-Red receives the packet with the MRT-Red label for the FEC, the
MRT transit forwarding functionality continues as described in MRT transit forwarding functionality continues as described in
Section 6.1.1.2. As with option 1A, the original FEC associated with Section 6.1.1.2. As with Option 1A, the original FEC associated with
the packet is maintained at each hop along the MRT. the packet is maintained at each hop along the MRT.
6.2.3. Other considerations for forwarding LDP traffic using MRT LDP 6.2.3. Other Considerations for Forwarding LDP Traffic Using MRT LDP
Labels Labels
Note that forwarding LDP traffic using MRT LDP Labels can be done Note that forwarding LDP traffic using MRT LDP Labels can be done
without the use of targeted LDP sessions when an MRT path to the without the use of targeted LDP sessions when an MRT path to the
destination FEC is used. The alternates selected in destination FEC is used. The alternates selected in [RFC7811] use
[I-D.ietf-rtgwg-mrt-frr-algorithm] use the MRT path to the the MRT path to the destination FEC, so targeted LDP sessions are not
destination FEC, so targeted LDP sessions are not needed. If instead needed. If instead one found it desirable to have the PLR use an MRT
one found it desirable to have the PLR use an MRT to reach the to reach the primary next-next-hop for the FEC, and then continue
primary next-next-hop for the FEC, and then continue forwarding the forwarding the LDP packet along the shortest path from the primary
LDP packet along the shortest path tree from the primary next-next- next-next-hop, this would require tunneling to the primary next-next-
hop, this would require tunneling to the primary next-next-hop and a hop and a targeted LDP session for the PLR to learn the FEC-label
targeted LDP session for the PLR to learn the FEC-label binding for binding for primary next-next-hop to correctly forward the packet.
primary next-next-hop to correctly forward the packet.
6.2.4. Required support for LDP traffic 6.2.4. Required Support for LDP Traffic
For greatest hardware compatibility, routers implementing MRT fast- For greatest hardware compatibility, routers implementing MRT fast
reroute of LDP traffic MUST support Option 1A of encoding the MT-ID reroute of LDP traffic MUST support Option 1A of encoding the MT-ID
in the labels (See Section 9). in the labels (See Section 9).
6.3. Forwarding IP Unicast Traffic over MRT Paths 6.3. Forwarding IP Unicast Traffic over MRT Paths
For IPv4 traffic, there is no currently practical alternative except For IPv4 traffic, there is no currently practical alternative except
tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
forwarding topology. For IPv6 traffic, in principle one could define forwarding topology. For IPv6 traffic, in principle, one could
bits in the IPv6 options header to indicate the MRT-Blue or MRT-Red define bits in the IPv6 options header to indicate the MRT-Blue or
forwarding topology. However, in this document, we have chosen not MRT-Red forwarding topology. However, in this document, we have
to define a solution that would work for IPv6 traffic but not for chosen not to define a solution that would work for IPv6 traffic but
IPv4 traffic. not for IPv4 traffic.
The choice of tunnel egress is flexible since any router closer to The choice of tunnel egress is flexible since any router closer to
the destination than the next-hop can work. This architecture the destination than the next hop can work. This architecture
assumes that the original destination in the area is selected (see assumes that the original destination in the area is selected (see
Section 11 for handling of multi-homed prefixes); another possible Section 11 for handling of multihomed prefixes); another possible
choice is the next-next-hop towards the destination. As discussed in choice is the next-next-hop towards the destination. As discussed in
the previous section, for LDP traffic, using the MRT to the original the previous section, for LDP traffic, using the MRT to the original
destination simplifies MRT-FRR by avoiding the need for targeted LDP destination simplifies MRT-FRR by avoiding the need for targeted LDP
sessions to the next-next-hop. For IP, that consideration doesn't sessions to the next-next-hop. For IP, that consideration doesn't
apply. apply.
Some situations require tunneling IP traffic along an MRT to a tunnel Some situations require tunneling IP traffic along an MRT to a tunnel
endpoint that is not the destination of the IP traffic. These endpoint that is not the destination of the IP traffic. These
situations will be discussed in detail later. We note here that an situations will be discussed in detail later. We note here that an
IP packet with a destination in a different IGP area/level from the IP packet with a destination in a different IGP area/level from the
PLR should be tunneled on the MRT to the Area Border Router (ABR) or PLR should be tunneled on the MRT to the Area Border Router (ABR) or
Level Border Router (LBR) on the shortest path to the destination. Level Border Router (LBR) on the shortest path to the destination.
For a destination outside of the PLR's MRT Island, the packet should For a destination outside of the PLR's MRT Island, the packet should
be tunneled on the MRT to a non-proxy-node immediately before the be tunneled on the MRT to a non-proxy-node immediately before the
named proxy-node on that particular color MRT. named proxy-node on that particular color MRT.
6.3.1. Tunneling IP traffic using MRT LDP Labels 6.3.1. Tunneling IP Traffic Using MRT LDP Labels
An IP packet can be tunneled along an MRT path by pushing the An IP packet can be tunneled along an MRT path by pushing the
appropriate MRT LDP label(s). Tunneling using LDP labels, as opposed appropriate MRT LDP label(s). Tunneling using LDP labels, as opposed
to IP headers, has the the advantage that more installed routers can to IP headers, has the advantage that more installed routers can do
do line-rate encapsulation and decapsulation using LDP than using IP. line-rate encapsulation and decapsulation using LDP than using IP.
Also, no additional IP addresses would need to be allocated or Also, no additional IP addresses would need to be allocated or
signaled. signaled.
6.3.1.1. Tunneling IP traffic using MRT LDP Label Option 1A 6.3.1.1. Tunneling IP Traffic Using MRT LDP Label Option 1A
The MRT LDP Label option 1A forwarding mechanism uses topology-scoped The MRT LDP Label Option 1A forwarding mechanism uses topology-scoped
FECs encoded using a single label as described in section FECs encoded using a single label as described in Section 6.1.1.1.
Section 6.1.1.1. When a PLR receives an IP packet that needs to be When a PLR receives an IP packet that needs to be forwarded on the
forwarded on the Red MRT to a particular tunnel endpoint, it does a MRT-Red to a particular tunnel endpoint, it does a label push
label push operation. The label pushed is the Red MRT label for a operation. The label pushed is the MRT-Red label for a FEC
FEC originated by the tunnel endpoint, learned from the next-hop on originated by the tunnel endpoint, learned from the next hop on the
the Red MRT. MRT-Red.
6.3.1.2. Tunneling IP traffic using MRT LDP Label Option 1B 6.3.1.2. Tunneling IP Traffic Using MRT LDP Label Option 1B
The MRT LDP Label option 1B forwarding mechanism encodes the topology The MRT LDP Label Option 1B forwarding mechanism encodes the topology
and the FEC using a two label stack as described in Section 6.1.1.2. and the FEC using a two-label stack as described in Section 6.1.1.2.
When a PLR receives an IP packet that needs to be forwarded on the When a PLR receives an IP packet that needs to be forwarded on the
Red MRT to a particular tunnel endpoint, the PLR pushes two labels on MRT-Red to a particular tunnel endpoint, the PLR pushes two labels on
the IP packet. The first (inner) label is the normal LDP label the IP packet. The first (inner) label is the normal LDP label
learned from the next-hop on the Red MRT, associated with a FEC learned from the next hop on the MRT-Red, associated with a FEC
originated by the tunnel endpoint. The second (outer) label is the originated by the tunnel endpoint. The second (outer) label is the
topology-identification label associated with the Red MRT. topology-id label associated with the MRT-Red.
For completeness, we note here a potential variation that uses a For completeness, we note here a potential variation that uses a
single label as opposed to two labels. In order to tunnel an IP single label as opposed to two labels. In order to tunnel an IP
packet over an MRT to the destination of the IP packet (as opposed to packet over an MRT to the destination of the IP packet as opposed to
an arbitrary tunnel endpoint), then we could just push a topology- an arbitrary tunnel endpoint, one could just push a topology-id label
identification label directly onto the packet. An MRT transit router directly onto the packet. An MRT transit router would need to pop
would need to pop the topology-id label, do an IP route lookup in the the topology-id label, do an IP route lookup in the context of that
context of that topology-id , and push the topology-id label. topology-id label, and push the topology-id label.
6.3.2. Tunneling IP traffic using MRT IP Tunnels 6.3.2. Tunneling IP Traffic Using MRT IP Tunnels
In order to tunnel over the MRT to a particular tunnel endpoint, the In order to tunnel over the MRT to a particular tunnel endpoint, the
PLR encapsulates the original IP packet with an additional IP header PLR encapsulates the original IP packet with an additional IP header
using the MRT-Blue or MRT-Red loopack address of the tunnel endpoint. using the MRT-Blue or MRT-Red loopback address of the tunnel
endpoint.
6.3.3. Required support for IP traffic 6.3.3. Required Support for IP Traffic
For greatest hardware compatibility and ease in removing the MRT- For greatest hardware compatibility and ease in removing the MRT-
topology marking at area/level boundaries, routers that support MPLS topology marking at area/level boundaries, routers that support MPLS
and implement IP MRT fast-reroute MUST support tunneling of IP and implement IP MRT fast reroute MUST support tunneling of IP
traffic using MRT LDP Label Option 1A (topology-scoped FEC encoded traffic using MRT LDP Label Option 1A (topology-scoped FEC encoded
using a single label). using a single label).
7. MRT Island Formation 7. MRT Island Formation
The purpose of communicating support for MRT is to indicate that the The purpose of communicating support for MRT is to indicate that the
MRT-Blue and MRT-Red forwarding topologies are created for transit MRT-Blue and MRT-Red forwarding topologies are created for transit
traffic. The MRT architecture allows for different, potentially traffic. The MRT architecture allows for different, potentially
incompatible options. In order to create consistent MRT forwarding incompatible options. In order to create consistent MRT forwarding
topologies, the routers participating in a particular MRT Island need topologies, the routers participating in a particular MRT Island need
skipping to change at page 17, line 9 skipping to change at page 17, line 9
interpretation defined here. interpretation defined here.
Other information needs to be communicated between routers for which Other information needs to be communicated between routers for which
there do not currently exist protocol extensions. This new there do not currently exist protocol extensions. This new
information needs to be shared among all routers in an IGP area, so information needs to be shared among all routers in an IGP area, so
defining extensions to existing IGPs to carry this information makes defining extensions to existing IGPs to carry this information makes
sense. These new protocol extensions will be defined elsewhere. sense. These new protocol extensions will be defined elsewhere.
Deployment scenarios using multi-topology OSPF or IS-IS, or running Deployment scenarios using multi-topology OSPF or IS-IS, or running
both IS-IS and OSPF on the same routers is out of scope for this both IS-IS and OSPF on the same routers is out of scope for this
specification. As with LFA, it is expected that OSPF Virtual Links specification. As with LFA, MRT-FRR does not support OSPF Virtual
will not be supported. Links.
At a high level, an MRT Island is defined as the set of routers At a high level, an MRT Island is defined as the set of routers
supporting the same MRT profile, in the same IGP area/level and the supporting the same MRT profile, in the same IGP area/level and with
bi-directional links interconnecting those routers. More detailed bidirectional links interconnecting those routers. More detailed
descriptions of these criteria are given below. descriptions of these criteria are given below.
7.1. IGP Area or Level 7.1. IGP Area or Level
All links in an MRT Island are bidirectional and belong to the same All links in an MRT Island are bidirectional and belong to the same
IGP area or level. For IS-IS, a link belonging to both level 1 and IGP area or level. For IS-IS, a link belonging to both Level-1 and
level 2 would qualify to be in multiple MRT Islands. A given ABR or Level-2 would qualify to be in multiple MRT Islands. A given ABR or
LBR can belong to multiple MRT Islands, corresponding to the areas or LBR can belong to multiple MRT Islands, corresponding to the areas or
levels in which it participates. Inter-area forwarding behavior is levels in which it participates. Inter-area forwarding behavior is
discussed in Section 10. discussed in Section 10.
7.2. Support for a specific MRT profile 7.2. Support for a Specific MRT Profile
All routers in an MRT Island support the same MRT profile. A router All routers in an MRT Island support the same MRT profile. A router
advertises support for a given MRT profile using an 8-bit MRT Profile advertises support for a given MRT profile using an 8-bit MRT Profile
ID value. The registry for the MRT Profile ID is defined in this ID value. The "MRT Profile Identifier Registry" is defined in this
document. The protocol extensions for advertising the MRT Profile ID document. The protocol extensions for advertising the MRT Profile ID
value will be defined elsewhere. A given router can support multiple value will be defined in a future specification. A given router can
MRT profiles and participate in multiple MRT Islands. The options support multiple MRT profiles and participate in multiple MRT
that make up an MRT profile, as well as the default MRT profile, are Islands. The options that make up an MRT Profile, as well as the
defined in Section 8. Default MRT Profile, are defined in Section 8.
The process of MRT Island formation takes place independently for The process of MRT Island formation takes place independently for
each MRT profile advertised by a given router. For example, consider each MRT profile advertised by a given router. For example, consider
a network with 40 connected routers in the same area advertising a network with 40 connected routers in the same area advertising
support for MRT Profile A and MRT Profile B. Two distinct MRT support for MRT Profile A and MRT Profile B. Two distinct MRT
Islands will be formed corresponding to Profile A and Profile B, with Islands will be formed corresponding to Profile A and Profile B, with
each island containing all 40 routers. A complete set of maximally each island containing all 40 routers. A complete set of maximally
redundant trees will be computed for each island following the rules redundant trees will be computed for each island following the rules
defined for each profile. If we add a third MRT Profile to this defined for each profile. If we add a third MRT Profile to this
example, with Profile C being advertised by a connected subset of 30 example, with Profile C being advertised by a connected subset of 30
routers, there will be a third MRT Island formed corresponding to routers, there will be a third MRT Island formed corresponding to
those 30 routers, and a third set of maximally redundant trees will those 30 routers, and a third set of maximally redundant trees will
be computed. In this example, 40 routers would compute and install be computed. In this example, 40 routers would compute and install
two sets of MRT transit forwarding entries corresponding to Profiles two sets of MRT transit forwarding entries corresponding to Profiles
A and B, while 30 routers would compute and install three sets of MRT A and B, while 30 routers would compute and install three sets of MRT
transit forwarding entries corresponding to Profiles A, B, and C. transit forwarding entries corresponding to Profiles A, B, and C.
7.3. Excluding additional routers and interfaces from the MRT Island 7.3. Excluding Additional Routers and Interfaces from the MRT Island
MRT takes into account existing IGP mechanisms for discouraging MRT takes into account existing IGP mechanisms for discouraging
traffic from using particular links and routers, and it introduces an traffic from using particular links and routers, and it introduces an
MRT-specific exclusion mechanism for links. MRT-specific exclusion mechanism for links.
7.3.1. Existing IGP exclusion mechanisms 7.3.1. Existing IGP Exclusion Mechanisms
Mechanisms for discouraging traffic from using particular links Mechanisms for discouraging traffic from using particular links
already exist in IS-IS and OSPF. In IS-IS, an interface configured already exist in IS-IS and OSPF. In IS-IS, an interface configured
with a metric of 2^24-2 (0xFFFFFE) will only be used as a last with a metric of 2^24-2 (0xFFFFFE) will only be used as a last
resort. (An interface configured with a metric of 2^24-1 (0xFFFFFF) resort. (An interface configured with a metric of 2^24-1 (0xFFFFFF)
will not be advertised into the topology.) In OSPF, an interface will not be advertised into the topology.) In OSPF, an interface
configured with a metric of 2^16-1 (0xFFFF) will only be used as a configured with a metric of 2^16-1 (0xFFFF) will only be used as a
last resort. These metrics can be configured manually to enforce last resort. These metrics can be configured manually to enforce
administrative policy, or they can be set in an automated manner as administrative policy or they can be set in an automated manner as
with LDP IGP synchronization [RFC5443]. with LDP IGP synchronization [RFC5443].
Mechanisms also already exist in IS-IS and OSPF to discourage or Mechanisms also already exist in IS-IS and OSPF to discourage or
prevent transit traffic from using a particular router. In IS-IS, prevent transit traffic from using a particular router. In IS-IS,
the overload bit is prevents transit traffic from using a router. the overload bit is prevents transit traffic from using a router.
For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing
interface metrics to 0xFFFF to discourage transit traffic from using interface metrics to 0xFFFF to discourage transit traffic from using
a router.( [RFC6987] defines the metric value 0xFFFF as a router. ([RFC6987] defines the metric value 0xFFFF as
MaxLinkMetric, a fixed architectural value for OSPF.) For OSPFv3, MaxLinkMetric, a fixed architectural value for OSPF.) For OSPFv3,
[RFC5340] specifies that a router be excluded from the intra-area [RFC5340] specifies that a router be excluded from the intra-area SPT
shortest path tree computation if the V6-bit or R-bit of the LSA computation if the V6-bit or R-bit of the Link State Advertisement
options is not set in the Router LSA. (LSA) options is not set in the Router LSA.
The following rules for MRT Island formation ensure that MRT FRR The following rules for MRT Island formation ensure that MRT FRR
protection traffic does not use a link or router that is discouraged protection traffic does not use a link or router that is discouraged
or prevented from carrying traffic by existing IGP mechanisms. or prevented from carrying traffic by existing IGP mechanisms.
1. A bidirectional link MUST be excluded from an MRT Island if 1. A bidirectional link MUST be excluded from an MRT Island if
either the forward or reverse cost on the link is 0xFFFFFE (for either the forward or reverse cost on the link is 0xFFFFFE (for
IS-IS) or 0xFFFF for OSPF. IS-IS) or 0xFFFF for OSPF.
2. A router MUST be excluded from an MRT Island if it is advertised 2. A router MUST be excluded from an MRT Island if it is advertised
with the overload bit set (for IS-IS), or it is advertised with with the overload bit set (for IS-IS), or it is advertised with
metric values of 0xFFFF on all of its outgoing interfaces (for metric values of 0xFFFF on all of its outgoing interfaces (for
OSPFv2 and OSPFv3). OSPFv2 and OSPFv3).
3. A router MUST be excluded from an MRT Island if it is advertised 3. A router MUST be excluded from an MRT Island if it is advertised
with either the V6-bit or R-bit of the LSA options not set in the with either the V6-bit or R-bit of the LSA options not set in the
Router LSA. Router LSA.
7.3.2. MRT-specific exclusion mechanism 7.3.2. MRT-Specific Exclusion Mechanism
This architecture also defines a means of excluding an otherwise This architecture also defines a means of excluding an otherwise
usable link from MRT Islands. The protocol extensions for usable link from MRT Islands. The protocol extensions for
advertising that a link is MRT-Ineligible will be defined elsewhere. advertising that a link is MRT-Ineligible will be defined elsewhere.
A link with either interface advertised as MRT-Ineligible MUST be A link with either interface advertised as MRT-Ineligible MUST be
excluded from an MRT Island. Note that an interface advertised as excluded from an MRT Island. Note that an interface advertised as
MRT-Ineligible by a router is ineligible with respect to all profiles MRT-Ineligible by a router is ineligible with respect to all profiles
advertised by that router. advertised by that router.
7.4. Connectivity 7.4. Connectivity
All of the routers in an MRT Island MUST be connected by All of the routers in an MRT Island MUST be connected by
bidirectional links with other routers in the MRT Island. bidirectional links with other routers in the MRT Island.
Disconnected MRT Islands will operate independently of one another. Disconnected MRT Islands will operate independently of one another.
7.5. Algorithm for MRT Island Identification 7.5. Algorithm for MRT Island Identification
An algorithm that allows a computing router to identify the routers An algorithm that allows a computing router to identify the routers
and links in the local MRT Island satisfying the above rules is given and links in the local MRT Island satisfying the above rules is given
in section 5.2 of [I-D.ietf-rtgwg-mrt-frr-algorithm]. in Section 5.2 of [RFC7811].
8. MRT Profile 8. MRT Profile
An MRT Profile is a set of values and options related to MRT An MRT Profile is a set of values and options related to MRT
behavior. The complete set of options is designated by the behavior. The complete set of options is designated by the
corresponding 8-bit Profile ID value. corresponding 8-bit Profile ID value.
This document specifies the values and options that correspond to the This document specifies the values and options that correspond to the
Default MRT Profile (Profile ID = 0). Future documents may define Default MRT Profile (Profile ID = 0). Future documents may define
other MRT Profiles by specifying the MRT Profile Options below. other MRT Profiles by specifying the MRT Profile Options below.
8.1. MRT Profile Options 8.1. MRT Profile Options
Below is a description of the values and options that define an MRT Below is a description of the values and options that define an MRT
Profile. Profile.
MRT Algorithm: This identifies the particular algorithm for MRT Algorithm: This identifies the particular algorithm for
computing maximally redundant trees used by the router for this computing maximally redundant trees used by the router for this
profile. profile.
MRT-Red MT-ID: This specifies the MPLS MT-ID to be associated with MRT-Red MT-ID: This specifies the MPLS MT-ID to be associated with
the MRT-Red forwarding topology. It is allocated from the MPLS the MRT-Red forwarding topology. It is allocated from the MPLS
Multi-Topology Identifiers Registry. Multi-Topology Identifiers Registry.
MRT-Blue MT-ID: This specifies the MPLS MT-ID to be associated with MRT-Blue MT-ID: This specifies the MPLS MT-ID to be associated with
the MRT-Blue forwarding topology. It is allocated from the MPLS the MRT-Blue forwarding topology. It is allocated from the MPLS
Multi-Topology Identifiers Registry. Multi-Topology Identifiers Registry.
GADAG Root Selection Policy: This specifies the manner in which the GADAG Root Selection Policy: This specifies the manner in which the
GADAG root is selected. All routers in the MRT island need to use GADAG root is selected. All routers in the MRT Island need to use
the same GADAG root in the calculations used construct the MRTs. the same GADAG root in the calculations used construct the MRTs.
A valid GADAG Root Selection Policy MUST be such that each router A valid GADAG Root Selection Policy MUST be such that each router
in the MRT island chooses the same GADAG root based on information in the MRT Island chooses the same GADAG root based on information
available to all routers in the MRT island. GADAG Root Selection available to all routers in the MRT Island. GADAG Root Selection
Priority values, advertised as router-specific MRT parameters, MAY Priority values, advertised as router-specific MRT parameters, MAY
be used in a GADAG Root Selection Policy. be used in a GADAG Root Selection Policy.
MRT Forwarding Mechanism: This specifies which forwarding mechanism MRT Forwarding Mechanism: This specifies which forwarding mechanism
the router uses to carry transit traffic along MRT paths. A the router uses to carry transit traffic along MRT paths. A
router which supports a specific MRT forwarding mechanism must router that supports a specific MRT forwarding mechanism must
program appropriate next-hops into the forwarding plane. The program appropriate next hops into the forwarding plane. The
current options are MRT LDP Label Option 1A, MRT LDP Label Option current options are MRT LDP Label Option 1A, MRT LDP Label Option
1B, IPv4 Tunneling, IPv6 Tunneling, and None. If IPv4 is 1B, IPv4 Tunneling, IPv6 Tunneling, and None. If IPv4 is
supported, then both MRT-Red and MRT-Blue IPv4 Loopback Addresses supported, then both MRT-Red and MRT-Blue IPv4 loopback addresses
SHOULD be specified. If IPv6 is supported, both MRT-Red and MRT- SHOULD be specified. If IPv6 is supported, both MRT-Red and MRT-
Blue IPv6 Loopback Addresses SHOULD be specified. Blue IPv6 loopback addresses SHOULD be specified.
Recalculation: Recalculation specifies the process and timing by Recalculation: Recalculation specifies the process and timing by
which new MRTs are computed after the topology has been modified. which new MRTs are computed after the topology has been modified.
Area/Level Border Behavior: This specifies how traffic traveling on Area/Level Border Behavior: This specifies how traffic traveling on
the MRT-Blue or MRT-Red in one area should be treated when it the MRT-Blue or MRT-Red in one area should be treated when it
passes into another area. passes into another area.
Other Profile-Specific Behavior: Depending upon the use-case for Other Profile-Specific Behavior: Depending upon the use-case for the
the profile, there may be additional profile-specific behavior. profile, there may be additional profile-specific behavior.
When a new MRT Profile is defined, new and unique values should be When a new MRT Profile is defined, new and unique values should be
allocated from the MPLS Multi-Topology Identifiers Registry, allocated from the "MPLS Multi-Topology Identifiers Registry",
corresponding to the MRT-Red and MRT-Blue MT-ID values for the new corresponding to the MRT-Red and MRT-Blue MT-ID values for the new
MRT Profile . MRT Profile.
If a router advertises support for multiple MRT profiles, then it If a router advertises support for multiple MRT profiles, then it
MUST create the transit forwarding topologies for each of those, MUST create the transit forwarding topologies for each of those,
unless the profile specifies the None option for MRT Forwarding unless the profile specifies the None option for the MRT Forwarding
Mechanism. Mechanism.
The ability of MRT-FRR to support transit forwarding entries for The ability of MRT-FRR to support transit forwarding entries for
multiple profiles can be used to facilitate a smooth transition from multiple profiles can be used to facilitate a smooth transition from
an existing deployed MRT Profile to a new MRT Profile. The new an existing deployed MRT Profile to a new MRT Profile. The new
profile can be activated in parallel with the existing profile, profile can be activated in parallel with the existing profile,
installing the transit forwarding entries for the new profile without installing the transit forwarding entries for the new profile without
affecting the transit forwarding entries for the existing profile. affecting the transit forwarding entries for the existing profile.
Once the new transit forwarding state has been verified, the router Once the new transit forwarding state has been verified, the router
can be configured to use the alternates computed by the new profile can be configured to use the alternates computed by the new profile
in the event of a failure. in the event of a failure.
8.2. Router-specific MRT paramaters 8.2. Router-Specific MRT Parameters
For some profiles, additional router-specific MRT parameters may need For some profiles, additional router-specific MRT parameters may need
to be advertised. While the set of options indicated by the MRT to be advertised. While the set of options indicated by the MRT
Profile ID must be identical for all routers in an MRT Island, these Profile ID must be identical for all routers in an MRT Island, these
router-specific MRT parameters may differ between routers in the same router-specific MRT parameters may differ between routers in the same
MRT island. Several such parameters are described below. MRT Island. Several such parameters are described below.
GADAG Root Selection Priority: A GADAG Root Selection Policy MAY GADAG Root Selection Priority: A GADAG Root Selection Policy MAY
rely on the GADAG Root Selection Priority values advertised by rely on the GADAG Root Selection Priority values advertised by
each router in the MRT island. A GADAG Root Selection Policy may each router in the MRT Island. A GADAG Root Selection Policy may
use the GADAG Root Selection Priority to allow network operators use the GADAG Root Selection Priority to allow network operators
to configure a parameter to ensure that the GADAG root is selected to configure a parameter to ensure that the GADAG root is selected
from a particular subset of routers. An example of this use of from a particular subset of routers. An example of this use of
the GADAG Root Selection Priority value by the GADAG Root the GADAG Root Selection Priority value by the GADAG Root
Selection Policy is given in the Default MRT profile below. Selection Policy is given in the Default MRT Profile below.
MRT-Red Loopback Address: This provides the router's loopback MRT-Red Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Red forwarding topology. address to reach the router via the MRT-Red forwarding topology.
It can be specified for either IPv4 or IPv6. Note that this It can be specified for either IPv4 or IPv6. Note that this
parameter is not needed to support the Default MRT profile. parameter is not needed to support the Default MRT Profile.
MRT-Blue Loopback Address: This provides the router's loopback MRT-Blue Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Blue forwarding topology. address to reach the router via the MRT-Blue forwarding topology.
It can be specified for either IPv4 and IPv6. Note that this It can be specified for either IPv4 and IPv6. Note that this
parameter is not needed to support the Default MRT profile. parameter is not needed to support the Default MRT Profile.
Protocol extensions for advertising a router's GADAG Root Selection Protocol extensions for advertising a router's GADAG Root Selection
Priority value will be defined in other documents. Protocol Priority value will be defined in other documents. Protocol
extensions for the advertising a router's MRT-Red and MRT-Blue extensions for the advertising a router's MRT-Red and MRT-Blue
Loopback Addresses will be defined elsewhere. loopback addresses will be defined elsewhere.
8.3. Default MRT profile 8.3. Default MRT Profile
The following set of options defines the default MRT Profile. The The following set of options defines the Default MRT Profile. The
default MRT profile is indicated by the MRT Profile ID value of 0. Default MRT Profile is indicated by the MRT Profile ID value of 0.
MRT Algorithm: MRT Lowpoint algorithm defined in MRT Algorithm: MRT Lowpoint algorithm defined in [RFC7811].
[I-D.ietf-rtgwg-mrt-frr-algorithm].
MRT-Red MPLS MT-ID: This value will be allocated from the MPLS MRT-Red MPLS MT-ID: This temporary registration has been allocated
Multi-Topology Identifiers Registry. The IANA request for this from the "MPLS Multi-Topology Identifiers" registry. The
allocation will be in another document. registration request appears in [LDP-MRT].
MRT-Blue MPLS MT-ID: This value will be allocated from the MPLS MRT-Blue MPLS MT-ID: This temporary registration has been allocated
Multi-Topology Identifiers Registry. The IANA request for this from the "MPLS Multi-Topology Identifiers" registry. The
allocation will be in another document. registration request appears in [LDP-MRT].
GADAG Root Selection Policy: Among the routers in the MRT Island GADAG Root Selection Policy: Among the routers in the MRT Island
with the lowest numerical value advertised for GADAG Root with the lowest numerical value advertised for GADAG Root
Selection Priority, an implementation MUST pick the router with Selection Priority, an implementation MUST pick the router with
the highest Router ID to be the GADAG root. Note that a lower the highest Router ID to be the GADAG root. Note that a lower
numerical value for GADAG Root Selection Priority indicates a numerical value for GADAG Root Selection Priority indicates a
higher preference for selection. higher preference for selection.
Forwarding Mechanisms: MRT LDP Label Option 1A Forwarding Mechanisms: MRT LDP Label Option 1A
Recalculation: Recalculation of MRTs SHOULD occur as described in Recalculation: Recalculation of MRTs SHOULD occur as described in
Section 12.2. This allows the MRT forwarding topologies to Section 12.2. This allows the MRT forwarding topologies to
support IP/LDP fast-reroute traffic. support IP/LDP fast-reroute traffic.
Area/Level Border Behavior: As described in Section 10, ABRs/LBRs Area/Level Border Behavior: As described in Section 10, ABRs/LBRs
SHOULD ensure that traffic leaving the area also exits the MRT-Red SHOULD ensure that traffic leaving the area also exits the MRT-Red
or MRT-Blue forwarding topology. or MRT-Blue forwarding topology.
9. LDP signaling extensions and considerations 9. LDP Signaling Extensions and Considerations
The protocol extensions for LDP will be defined in another document. The protocol extensions for LDP will be defined in another document.
A router must indicate that it has the ability to support MRT; having A router must indicate that it has the ability to support MRT; having
this explicit allows the use of MRT-specific processing, such as this explicit allows the use of MRT-specific processing, such as
special handling of FECs sent with the Rainbow MRT MT-ID. special handling of FECs sent with the Rainbow MRT MT-ID.
A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies
to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles. to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles.
The FEC-label bindings for the default shortest-path based MT-ID 0 The FEC-label bindings for the default shortest-path-based MT-ID 0
MUST still be sent (even though it could be inferred from the Rainbow MUST still be sent (even though it could be inferred from the Rainbow
FEC-label bindings) to ensure continuous operation of normal LDP FEC-label bindings) to ensure continuous operation of normal LDP
forwarding. The Rainbow MRT MT-ID is defined to provide an easy way forwarding. The Rainbow MRT MT-ID is defined to provide an easy way
to handle the special signaling that is needed at ABRs or LBRs. It to handle the special signaling that is needed at ABRs or LBRs. It
avoids the problem of needing to signal different MPLS labels to avoids the problem of needing to signal different MPLS labels to
different LDP neighbors for the same FEC. Because the Rainbow MRT different LDP neighbors for the same FEC. Because the Rainbow MRT
MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not
MRT profile specific. MRT profile specific.
The value of the Rainbow MRT MPLS MT-ID will be allocated from the The value of the Rainbow MRT MPLS MT-ID has been temporarily
MPLS Multi-Topology Identifiers Registry. The IANA request for this allocated from the "MPLS Multi-Topology Identifiers" registry. The
allocation will be in another document. registration request appears in [LDP-MRT].
10. Inter-area Forwarding Behavior 10. Inter-area Forwarding Behavior
An ABR/LBR has two forwarding roles. First, it forwards traffic An ABR/LBR has two forwarding roles. First, it forwards traffic
within areas. Second, it forwards traffic from one area into within areas. Second, it forwards traffic from one area into
another. These same two roles apply for MRT transit traffic. another. These same two roles apply for MRT transit traffic.
Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
on MRT-Red or MRT-Blue in that area. However, it is desirable for on MRT-Red or MRT-Blue in that area. However, it is desirable for
traffic leaving the area to also exit MRT-Red or MRT-Blue and return traffic leaving the area to also exit MRT-Red or MRT-Blue and return
to shortest path forwarding. to shortest path forwarding.
skipping to change at page 23, line 32 skipping to change at page 23, line 40
interface that the packet was received on. Otherwise, the packet interface that the packet was received on. Otherwise, the packet
should be removed from MRT-Red or MRT-Blue and forwarded on the should be removed from MRT-Red or MRT-Blue and forwarded on the
shortest-path default forwarding topology. shortest-path default forwarding topology.
The above description applies to OSPF. The same essential behavior The above description applies to OSPF. The same essential behavior
also applies to IS-IS if one substitutes IS-IS level for OSPF area. also applies to IS-IS if one substitutes IS-IS level for OSPF area.
However, the analogy with OSPF is not exact. An interface in OSPF However, the analogy with OSPF is not exact. An interface in OSPF
can only be in one area, whereas an interface in IS-IS can be in both can only be in one area, whereas an interface in IS-IS can be in both
Level-1 and Level-2. Therefore, to avoid confusion and address this Level-1 and Level-2. Therefore, to avoid confusion and address this
difference, we explicitly describe the behavior for IS-IS in difference, we explicitly describe the behavior for IS-IS in
Appendix A. In the following sections only the OSPF terminology is Appendix A. In the following sections, only the OSPF terminology is
used. used.
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A 10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A
For LDP forwarding where a single label specifies (MT-ID, FEC), the For LDP forwarding where a single label specifies (MT-ID, FEC), the
ABR is responsible for advertising the proper label to each neighbor. ABR is responsible for advertising the proper label to each neighbor.
Assume that an ABR has allocated three labels for a particular Assume that an ABR has allocated three labels for a particular
destination; those labels are L_primary, L_blue, and L_red. To those destination: L_primary, L_blue, and L_red. To those routers in the
routers in the same area as the best route to the destination, the same area as the best route to the destination, the ABR advertises
ABR advertises the following FEC-label bindings: L_primary for the the following FEC-label bindings: L_primary for the default topology,
default topology, L_blue for the MRT-Blue MT-ID and L_red for the L_blue for the MRT-Blue MT-ID, and L_red for the MRT-Red MT-ID, as
MRT-Red MT-ID, as expected. However, to routers in other areas, the expected. However, to routers in other areas, the ABR advertises the
ABR advertises the following FEC-label bindings: L_primary for the following FEC-label bindings: L_primary for the default topology and
default topology, and L_primary for the Rainbow MRT MT-ID. L_primary for the Rainbow MRT MT-ID. Associating L_primary with the
Associating L_primary with the Rainbow MRT MT-ID causes the receiving Rainbow MRT MT-ID causes the receiving routers to use L_primary for
routers to use L_primary for the MRT-Blue MT-ID and for the MRT-Red the MRT-Blue MT-ID and for the MRT-Red MT-ID.
MT-ID.
The ABR installs all next-hops for the best area: primary next-hops The ABR installs all next hops for the best area: primary next hops
for L_primary, MRT-Blue next-hops for L_blue, and MRT-Red next-hops for L_primary, MRT-Blue next hops for L_blue, and MRT-Red next hops
for L_red. Because the ABR advertised (Rainbow MRT MT-ID, FEC) with for L_red. Because the ABR advertised (Rainbow MRT MT-ID, FEC) with
L_primary to neighbors not in the best area, packets from those L_primary to neighbors not in the best area, packets from those
neighbors will arrive at the ABR with a label L_primary and will be neighbors will arrive at the ABR with a label L_primary and will be
forwarded into the best area along the default topology. By forwarded into the best area along the default topology. By
controlling what labels are advertised, the ABR can thus enforce that controlling what labels are advertised, the ABR can thus enforce that
packets exiting the area do so on the shortest-path default topology. packets exiting the area do so on the shortest-path default topology.
10.1.1. Motivation for Creating the Rainbow-FEC 10.1.1. Motivation for Creating the Rainbow-FEC
The desired forwarding behavior could be achieved in the above The desired forwarding behavior could be achieved in the above
skipping to change at page 24, line 28 skipping to change at page 24, line 32
the ABR advertise the following FEC-label bindings to neighbors not the ABR advertise the following FEC-label bindings to neighbors not
in the best area: L1_primary for the default topology, L1_primary for in the best area: L1_primary for the default topology, L1_primary for
the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing this the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing this
would require machinery to spoof the labels used in FEC-label binding would require machinery to spoof the labels used in FEC-label binding
advertisements on a per-neighbor basis. Such label-spoofing advertisements on a per-neighbor basis. Such label-spoofing
machinery does not currently exist in most LDP implementations and machinery does not currently exist in most LDP implementations and
doesn't have other obvious uses. doesn't have other obvious uses.
Many existing LDP implementations do however have the ability to Many existing LDP implementations do however have the ability to
filter FEC-label binding advertisements on a per-neighbor basis. The filter FEC-label binding advertisements on a per-neighbor basis. The
Rainbow-FEC allows us to re-use the existing per-neighbor FEC Rainbow-FEC allows us to reuse the existing per-neighbor FEC
filtering machinery to achieve the desired result. By introducing filtering machinery to achieve the desired result. By introducing
the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
advertise the FEC-label binding for the Rainbow-FEC (and filter those advertise the FEC-label binding for the Rainbow-FEC (and filter those
for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR. for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR.
An ABR may choose to either advertise the Rainbow-FEC or advertise An ABR may choose to either distribute the Rainbow-FEC or distribute
separate MRT-Blue and MRT-Red advertisements. This is a local separate MRT-Blue and MRT-Red advertisements. This is a local
choice. A router that supports the MRT LDP Label Option 1A choice. A router that supports the MRT LDP Label Option 1A
Forwarding Mechanism MUST be able to receive and correctly interpret forwarding mechanism MUST be able to receive and correctly interpret
the Rainbow-FEC. the Rainbow-FEC.
10.2. ABR Forwarding Behavior with IP Tunneling (option 2) 10.2. ABR Forwarding Behavior with IP Tunneling (Option 2)
If IP tunneling is used, then the ABR behavior is dependent upon the If IP tunneling is used, then the ABR behavior is dependent upon the
outermost IP address. If the outermost IP address is an MRT loopback outermost IP address. If the outermost IP address is an MRT loopback
address of the ABR, then the packet is decapsulated and forwarded address of the ABR, then the packet is decapsulated and forwarded
based upon the inner IP address, which should go on the default SPT based upon the inner IP address, which should go on the default SPT
topology. If the outermost IP address is not an MRT loopback address topology. If the outermost IP address is not an MRT loopback address
of the ABR, then the packet is simply forwarded along the associated of the ABR, then the packet is simply forwarded along the associated
forwarding topology. A PLR sending traffic to a destination outside forwarding topology. A PLR sending traffic to a destination outside
its local area/level will pick the MRT and use the associated MRT its local area/level will pick the MRT and use the associated MRT
loopback address of the selected ABR advertising the lowest cost to loopback address of the selected ABR advertising the lowest cost to
the external destination. the external destination.
Thus, for these two MRT Forwarding Mechanisms (MRT LDP Label option Thus, for these two MRT forwarding mechanisms (MRT LDP Label Option
1A and IP tunneling option 2), there is no need for additional 1A and IP tunneling Option 2), there is no need for additional
computation or per-area forwarding state. computation or per-area forwarding state.
10.3. ABR Forwarding Behavior with MRT LDP Label option 1B 10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B
The other MRT forwarding mechanism described in Section 6 uses two The other MRT forwarding mechanism described in Section 6 uses two
labels, a topology-id label, and a FEC-label. This mechanism would labels: a topology-id label and a FEC-label. This mechanism would
require that any router whose MRT-Red or MRT-Blue next-hop is an ABR require that any router whose MRT-Red or MRT-Blue next hop is an ABR
would need to determine whether the ABR would forward the packet out would need to determine whether the ABR would forward the packet out
of the area/level. If so, then that router should pop off the of the area/level. If so, then that router should pop off the
topology-identification label before forwarding the packet to the topology-id label before forwarding the packet to the ABR.
ABR.
For example, in Figure 3, if node H fails, node E has to put traffic For example, in Figure 3, if node H fails, node E has to put traffic
towards prefix p onto MRT-Red. But since node D knows that ABR1 will towards prefix p onto MRT-Red. But since node D knows that ABR1 will
use a best route from another area, it is safe for D to pop the use a best route from another area, it is safe for D to pop the
Topology-Identification Label and just forward the packet to ABR1 topology-id label and just forward the packet to ABR1 along the MRT-
along the MRT-Red next-hop. ABR1 will use the shortest path in Area Red next hop. ABR1 will use the shortest path in Area 10.
10.
In all cases for IS-IS and most cases for OSPF, the penultimate In all cases for IS-IS and most cases for OSPF, the penultimate
router can determine what decision the adjacent ABR will make. The router can determine what decision the adjacent ABR will make. The
one case where it can't be determined is when two ASBRs are in one case where it can't be determined is when two ASBRs are in
different non-backbone areas attached to the same ABR, then the different non-backbone areas attached to the same ABR, then the
ASBR's Area ID may be needed for tie-breaking (prefer the route with ASBR's Area ID may be needed for tie-breaking (prefer the route with
the largest OPSF area ID) and the Area ID isn't announced as part of the largest OSPF area ID), and the Area ID isn't announced as part of
the ASBR link-state advertisement (LSA). In this one case, the ASBR LSA. In this one case, suboptimal forwarding along the MRT
suboptimal forwarding along the MRT in the other area would happen. in the other area would happen. If that becomes a realistic
If that becomes a realistic deployment scenario, protocol extensions deployment scenario, protocol extensions could be developed to
could be developed to address this issue. address this issue.
+----[C]---- --[D]--[E] --[D]--[E] +----[C]---- --[D]--[E] --[D]--[E]
| \ / \ / \ | \ / \ / \
p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+ p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+
| / \ / | \ / | | / \ / | \ / |
+----[B]---- --[F]--[G] | --[F]--[G] | +----[B]---- --[F]--[G] | --[F]--[G] |
| | | |
| other | | other |
+----------[p]-------+ +----------[p]-------+
area area
skipping to change at page 26, line 36 skipping to change at page 26, line 36
->[D]->[E] -<[D]<-[E] ->[D]->[E] -<[D]<-[E]
/ \ / \ / \ / \
[ABR1] Area 0 [H]-+ +-[ABR1] [H] [ABR1] Area 0 [H]-+ +-[ABR1] [H]
/ | | \ / | | \
[F]->[G] V V -<[F]<-[G] [F]->[G] V V -<[F]<-[G]
| | | |
| | | |
[p]<------+ +--------->[p] [p]<------+ +--------->[p]
(d) Blue MRT in Area 0 (e) Red MRT in Area 0 (d) MRT-Blue in Area 0 (e) MRT-Red in Area 0
Figure 3: ABR Forwarding Behavior and MRTs Figure 3: ABR Forwarding Behavior and MRTs
11. Prefixes Multiply Attached to the MRT Island 11. Prefixes Multiply Attached to the MRT Island
How a computing router S determines its local MRT Island for each How a computing router S determines its local MRT Island for each
supported MRT profile is already discussed in Section 7. supported MRT profile is already discussed in Section 7.
There are two types of prefixes or FECs that may be multiply attached There are two types of prefixes or FECs that may be multiply attached
to an MRT Island. The first type are multi-homed prefixes that to an MRT Island. The first type are multihomed prefixes that
usually connect at a domain or protocol boundary. The second type usually connect at a domain or protocol boundary. The second type
represent routers that do not support the profile for the MRT Island. represent routers that do not support the profile for the MRT Island.
The key difference is whether the traffic, once out of the MRT The key difference is whether the traffic, once out of the MRT
Island, might re-enter the MRT Island if a loop-free exit point is Island, might re-enter the MRT Island if a loop-free exit point is
not selected. not selected.
FRR using LFA has the useful property that it is able to protect FRR using LFA has the useful property that it is able to protect
multi-homed prefixes against ABR failure. For instance, if a prefix multihomed prefixes against ABR failure. For instance, if a prefix
from the backbone is available via both ABR A and ABR B, if A fails, from the backbone is available via both ABR A and ABR B, if A fails,
then the traffic should be redirected to B. This can be accomplished then the traffic should be redirected to B. This can be accomplished
with MRT FRR as well. with MRT FRR as well.
If ASBR protection is desired, this has additional complexities if If ASBR protection is desired, this has additional complexities if
the ASBRs are in different areas. Similarly, protecting labeled BGP the ASBRs are in different areas. Similarly, protecting labeled BGP
traffic in the event of an ASBR failure has additional complexities traffic in the event of an ASBR failure has additional complexities
due to the per-ASBR label spaces involved. due to the per-ASBR label spaces involved.
As discussed in [RFC5286], a multi-homed prefix could be: As discussed in [RFC5286], a multihomed prefix could be:
o An out-of-area prefix announced by more than one ABR, o An out-of-area prefix announced by more than one ABR,
o An AS-External route announced by 2 or more ASBRs, o An AS-External route announced by two or more ASBRs,
o A prefix with iBGP multipath to different ASBRs, o A prefix with iBGP multipath to different ASBRs,
o etc. o etc.
See Appendix B for a discussion of a general issue with multi-homed See Appendix B for a discussion of a general issue with multihomed
prefixes connected in two different areas. prefixes connected in two different areas.
There are also two different approaches to protection. The first is There are also two different approaches to protection. The first is
tunnel endpoint selection where the PLR picks a router to tunnel to tunnel endpoint selection where the PLR picks a router to tunnel to
where that router is loop-free with respect to the failure-point. where that router is loop-free with respect to the failure-point.
Conceptually, the set of candidate routers to provide LFAs expands to Conceptually, the set of candidate routers to provide LFAs expands to
all routers that can be reached via an MRT alternate, attached to the all routers that can be reached via an MRT alternate, attached to the
prefix. prefix.
The second is to use a proxy-node, that can be named via MPLS label The second is to use a proxy-node, which can be named via MPLS label
or IP address, and pick the appropriate label or IP address to reach or IP address, and pick the appropriate label or IP address to reach
it on either MRT-Blue or MRT-Red as appropriate to avoid the failure it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
point. A proxy-node can represent a destination prefix that can be point. A proxy-node can represent a destination prefix that can be
attached to the MRT Island via at least two routers. It is termed a attached to the MRT Island via at least two routers. It is termed a
named proxy-node if there is a way that traffic can be encapsulated named proxy-node if there is a way that traffic can be encapsulated
to reach specifically that proxy-node; this could be because there is to reach specifically that proxy-node; this could be because there is
an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue
IP addresses are advertised (in an as-yet undefined fashion) for that IP addresses are advertised (in an as-yet undefined fashion) for that
proxy-node. Traffic to a named proxy-node may take a different path proxy-node. Traffic to a named proxy-node may take a different path
than traffic to the attaching router; traffic is also explicitly than traffic to the attaching router; traffic is also explicitly
forwarded from the attaching router along a predetermined interface forwarded from the attaching router along a predetermined interface
towards the relevant prefixes. towards the relevant prefixes.
For IP traffic, multi-homed prefixes can use tunnel endpoint For IP traffic, multihomed prefixes can use tunnel endpoint
selection. For IP traffic that is destined to a router outside the selection. For IP traffic that is destined to a router outside the
MRT Island, if that router is the egress for a FEC advertised into MRT Island, if that router is the egress for a FEC advertised into
the MRT Island, then the named proxy-node approach can be used. the MRT Island, then the named proxy-node approach can be used.
For LDP traffic, there is always a FEC advertised into the MRT For LDP traffic, there is always a FEC advertised into the MRT
Island. The named proxy-node approach should be used, unless the Island. The named proxy-node approach should be used, unless the
computing router S knows the label for the FEC at the selected tunnel computing router S knows the label for the FEC at the selected tunnel
endpoint. endpoint.
If a FEC is advertised from outside the MRT Island into the MRT If a FEC is advertised from outside the MRT Island into the MRT
Island and the forwarding mechanism specified in the profile includes Island and the forwarding mechanism specified in the profile includes
LDP, then the routers learning that FEC MUST also advertise labels LDP Label Option 1A, then the routers learning that FEC MUST also
for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT advertise labels for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors
Island. Any router receiving a FEC corresponding to a router outside inside the MRT Island. Any router receiving a FEC corresponding to a
the MRT Island or to a multi-homed prefix MUST compute and install router outside the MRT Island or to a multihomed prefix MUST compute
the transit MRT-Blue and MRT-Red next-hops for that FEC. The FEC- and install the transit MRT-Blue and MRT-Red next hops for that FEC.
label bindings for the topology-scoped FECs ((MT-ID 0, FEC), (MRT- The FEC-label bindings for the topology-scoped FECs ((MT-ID 0, FEC),
Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to (MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
neighbors inside the MRT Island. neighbors inside the MRT Island.
11.1. Protecting Multi-Homed Prefixes using Tunnel Endpoint Selection 11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint Selection
Tunnel endpoint selection is a local matter for a router in the MRT Tunnel endpoint selection is a local matter for a router in the MRT
Island since it pertains to selecting and using an alternate and does Island since it pertains to selecting and using an alternate and does
not affect the transit MRT-Red and MRT-Blue forwarding topologies. not affect the transit MRT-Red and MRT-Blue forwarding topologies.
Let the computing router be S and the next-hop F be the node whose Let the computing router be S and the next hop F be the node whose
failure is to be avoided. Let the destination be prefix p. Have A failure is to be avoided. Let the destination be prefix p. Have A
be the router to which the prefix p is attached for S's shortest path be the router to which the prefix p is attached for S's shortest path
to p. to p.
The candidates for tunnel endpoint selection are those to which the The candidates for tunnel endpoint selection are those to which the
destination prefix is attached in the area/level. For a particular destination prefix is attached in the area/level. For a particular
candidate B, it is necessary to determine if B is loop-free to reach candidate B, it is necessary to determine if B is loop-free to reach
p with respect to S and F for node-protection or at least with p with respect to S and F for node-protection or at least with
respect to S and the link (S, F) for link-protection. If B will respect to S and the link (S, F) for link-protection. If B will
always prefer to send traffic to p via a different area/level, then always prefer to send traffic to p via a different area/level, then
skipping to change at page 29, line 7 skipping to change at page 29, line 7
to that given in [RFC5286]. In the inequalities below, D_opt(X,Y) to that given in [RFC5286]. In the inequalities below, D_opt(X,Y)
means the shortest distance from node X to node Y, and D_opt(X,p) means the shortest distance from node X to node Y, and D_opt(X,p)
means the shortest distance from node X to prefix p. means the shortest distance from node X to prefix p.
Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p) Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p) Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)
The latter is equivalent to the following, which avoids the need to The latter is equivalent to the following, which avoids the need to
compute the shortest path from F to p. compute the shortest path from F to p.
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, F)
F)
Finally, the rules for Endpoint selection are given below. The basic Finally, the rules for Endpoint selection are given below. The basic
idea is to repair to the prefix-advertising router selected for the idea is to repair to the prefix-advertising router selected for the
shortest-path and only to select and tunnel to a different endpoint shortest-path and only to select and tunnel to a different endpoint
if necessary (e.g. A=F or F is a cut-vertex or the link (S,F) is a if necessary (e.g., A=F or F is a cut-vertex or the link (S,F) is a
cut-link). cut-link).
1. Does S have a node-protecting alternate to A? If so, select 1. Does S have a node-protecting alternate to A? If so, select
that. Tunnel the packet to A along that alternate. For example, that. Tunnel the packet to A along that alternate. For example,
if LDP is the forwarding mechanism, then push the label (MRT-Red, if LDP is the forwarding mechanism, then push the label (MRT-Red,
A) or (MRT-Blue, A) onto the packet. A) or (MRT-Blue, A) onto the packet.
2. If not, then is there a router B that is loop-free to reach p 2. If not, then is there a router B that is loop-free to reach p
while avoiding both F and S? If so, select B as the end-point. while avoiding both F and S? If so, select B as the endpoint.
Determine the MRT alternate to reach B while avoiding F. Tunnel Determine the MRT alternate to reach B while avoiding F. Tunnel
the packet to B along that alternate. For example, with LDP, the packet to B along that alternate. For example, with LDP,
push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet. push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.
3. If not, then does S have a link-protecting alternate to A? If 3. If not, then does S have a link-protecting alternate to A? If
so, select that. so, select that.
4. If not, then is there a router B that is loop-free to reach p 4. If not, then is there a router B that is loop-free to reach p
while avoiding S and the link from S to F? If so, select B as while avoiding S and the link from S to F? If so, select B as
the endpoint and the MRT alternate for reaching B from S that the endpoint and the MRT alternate for reaching B from S that
avoid the link (S,F). avoid the link (S,F).
The tunnel endpoint selected will receive a packet destined to itself The tunnel endpoint selected will receive a packet destined to itself
and, being the egress, will pop that MPLS label (or have signaled and, being the egress, will pop that MPLS label (or have signaled
Implicit Null) and forward based on what is underneath. This Implicit Null) and forward based on what is underneath. This
suffices for IP traffic since the tunnel endpoint can use the IP suffices for IP traffic since the tunnel endpoint can use the IP
header of the original packet to continue forwarding the packet. header of the original packet to continue forwarding the packet.
However, tunnelling of LDP traffic requires targeted LDP sessions for However, tunneling of LDP traffic requires targeted LDP sessions for
learning the FEC-label binding at the tunnel endpoint. learning the FEC-label binding at the tunnel endpoint.
11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes
Instead, the named proxy-node method works with LDP traffic without Instead, the named proxy-node method works with LDP traffic without
the need for targeted LDP sessions. It also has a clear advantage the need for targeted LDP sessions. It also has a clear advantage
over tunnel endpoint selection, in that it is possible to explicitly over tunnel endpoint selection, in that it is possible to explicitly
forward from the MRT Island along an interface to a loop-free island forward from the MRT Island along an interface to a loop-free island
neighbor when that interface may not be a primary next-hop. neighbor when that interface may not be a primary next hop.
A named proxy-node represents one or more destinations and, for LDP A named proxy-node represents one or more destinations and, for LDP
forwarding, has a FEC associated with it that is signalled into the forwarding, has a FEC associated with it that is signaled into the
MRT Island. Therefore, it is possible to explicitly label packets to MRT Island. Therefore, it is possible to explicitly label packets to
go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT
Island, the label will swap to meaning (MT-ID 0, FEC). It would be Island, the label will swap to meaning (MT-ID 0, FEC). It would be
possible to have named proxy-nodes for IP forwarding, but this would possible to have named proxy-nodes for IP forwarding, but this would
require extensions to signal two IP addresses to be associated with require extensions to signal two IP addresses to be associated with
MRT-Red and MRT-Blue for the proxy-node. A named proxy-node can be MRT-Red and MRT-Blue for the proxy-node. A named proxy-node can be
uniquely represented by the two routers in the MRT Island to which it uniquely represented by the two routers in the MRT Island to which it
is connected. The extensions to signal such IP addresses will be is connected. The extensions to signal such IP addresses will be
defined elsewhere. The details of what label-bindings must be defined elsewhere. The details of what label-bindings must be
originated will be described in another document. originated will be described in another document.
Computing the MRT next-hops to a named proxy-node and the MRT Computing the MRT next hops to a named proxy-node and the MRT
alternate for the computing router S to avoid a particular failure alternate for the computing router S to avoid a particular failure
node F is straightforward. The details of the simple constant-time node F is straightforward. The details of the simple constant-time
functions, Select_Proxy_Node_NHs() and functions, Select_Proxy_Node_NHs() and
Select_Alternates_Proxy_Node(), are given in Select_Alternates_Proxy_Node(), are given in [RFC7811]. A key point
[I-D.ietf-rtgwg-mrt-frr-algorithm]. A key point is that computing is that computing these MRT next hops and alternates can be done as
these MRT next-hops and alternates can be done as new named proxy- new named proxy-nodes are added or removed without requiring a new
nodes are added or removed without requiring a new MRT computation or MRT computation or impacting other existing MRT paths. This maps
impacting other existing MRT paths. This maps very well to, for very well to, for example, how OSPFv2 (see [RFC2328], Section 16.5)
example, how OSPFv2 (see [RFC2328] Section 16.5) does incremental does incremental updates for new summary-LSAs.
updates for new summary-LSAs.
The remaining question is how to attach the named proxy-node to the The remaining question is how to attach the named proxy-node to the
MRT Island; all the routers in the MRT Island MUST do this MRT Island; all the routers in the MRT Island MUST do this
consistently. No more than 2 routers in the MRT Island can be consistently. No more than two routers in the MRT Island can be
selected; one should only be selected if there are no others that selected; one should only be selected if there are no others that
meet the necessary criteria. The named proxy-node is logically part meet the necessary criteria. The named proxy-node is logically part
of the area/level. of the area/level.
There are two sources for candidate routers in the MRT Island to There are two sources for candidate routers in the MRT Island to
connect to the named proxy-node. The first set are those routers in connect to the named proxy-node. The first set is made up of those
the MRT Island that are advertising the prefix; the named-proxy-cost routers in the MRT Island that are advertising the prefix; the named-
assigned to each prefix-advertising router is the announced cost to proxy-cost assigned to each prefix-advertising router is the
the prefix. The second set are those routers in the MRT Island that announced cost to the prefix. The second set is made up of those
are connected to routers not in the MRT Island but in the same area/ routers in the MRT Island that are connected to routers not in the
level; such routers will be defined as Island Border Routers (IBRs). MRT Island but in the same area/level; such routers will be defined
The routers connected to the IBRs that are not in the MRT Island and as Island Border Routers (IBRs). The routers connected to the IBRs
are in the same area/level as the MRT island are Island that are not in the MRT Island and are in the same area/level as the
Neighbors(INs). MRT Island are Island Neighbors (INs).
Since packets sent to the named proxy-node along MRT-Red or MRT-Blue Since packets sent to the named proxy-node along MRT-Red or MRT-Blue
may come from any router inside the MRT Island, it is necessary that may come from any router inside the MRT Island, it is necessary that
whatever router to which an IBR forwards the packet be loop-free with whatever router to which an IBR forwards the packet be loop-free with
respect to the whole MRT Island for the destination. Thus, an IBR is respect to the whole MRT Island for the destination. Thus, an IBR is
a candidate router only if it possesses at least one IN whose a candidate router only if it possesses at least one IN whose
shortest path to the prefix does not enter the MRT Island. A method shortest path to the prefix does not enter the MRT Island. A method
for identifying loop-free Island Neighbors(LFINs) is given in for identifying Loop-Free Island Neighbors (LFINs) is given in
[I-D.ietf-rtgwg-mrt-frr-algorithm]. The named-proxy-cost assigned to [RFC7811]. The named-proxy-cost assigned to each (IBR, IN) pair is
each (IBR, IN) pair is cost(IBR, IN) + D_opt(IN, prefix). cost(IBR, IN) + D_opt(IN, prefix).
From the set of prefix-advertising routers and the set of IBRs with From the set of prefix-advertising routers and the set of IBRs with
at least one LFIN, the two routers with the lowest named-proxy-cost at least one LFIN, the two routers with the lowest named-proxy-cost
are selected. Ties are broken based upon the lowest Router ID. For are selected. Ties are broken based upon the lowest Router ID. For
ease of discussion, the two selected routers will be referred to as ease of discussion, the two selected routers will be referred to as
proxy-node attachment routers. proxy-node attachment routers.
A proxy-node attachment router has a special forwarding role. When a A proxy-node attachment router has a special forwarding role. When a
packet is received destined to (MRT-Red, prefix) or (MRT-Blue, packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
prefix), if the proxy-node attachment router is an IBR, it MUST swap prefix), if the proxy-node attachment router is an IBR, it MUST swap
to the shortest path forwarding topology (e.g. swap to the label for to the shortest path forwarding topology (e.g., swap to the label for
(MT-ID 0, prefix) or remove the outer IP encapsulation) and forward (MT-ID 0, prefix) or remove the outer IP encapsulation) and forward
the packet to the IN whose cost was used in the selection. If the the packet to the IN whose cost was used in the selection. If the
proxy-node attachment router is not an IBR, then the packet MUST be proxy-node attachment router is not an IBR, then the packet MUST be
removed from the MRT forwarding topology and sent along the removed from the MRT forwarding topology and sent along the
interface(s) that caused the router to advertise the prefix; this interface(s) that caused the router to advertise the prefix; this
interface might be out of the area/level/AS. interface might be out of the area/level/AS.
11.3. MRT Alternates for Destinations Outside the MRT Island 11.3. MRT Alternates for Destinations outside the MRT Island
A natural concern with new functionality is how to have it be useful A natural concern with new functionality is how to have it be useful
when it is not deployed across an entire IGP area. In the case of when it is not deployed across an entire IGP area. In the case of
MRT FRR, where it provides alternates when appropriate LFAs aren't MRT FRR, where it provides alternates when appropriate LFAs aren't
available, there are also deployment scenarios where it may make available, there are also deployment scenarios where it may make
sense to only enable some routers in an area with MRT FRR. A simple sense to only enable some routers in an area with MRT FRR. A simple
example of such a scenario would be a ring of 6 or more routers that example of such a scenario would be a ring of six or more routers
is connected via two routers to the rest of the area. that is connected via two routers to the rest of the area.
Destinations inside the local island can obviously use MRT Destinations inside the local island can obviously use MRT
alternates. Destinations outside the local island can be treated alternates. Destinations outside the local island can be treated
like a multi-homed prefix and either Endpoint Selection or Named like a multihomed prefix and either endpoint selection or Named
Proxy-Nodes can be used. Named Proxy-Nodes MUST be supported when Proxy-Nodes can be used. Named proxy-nodes MUST be supported when
LDP forwarding is supported and a label-binding for the destination LDP forwarding is supported and a label-binding for the destination
is sent to an IBR. is sent to an IBR.
Naturally, there are more complicated options to improve coverage, Naturally, there are more-complicated options to improve coverage,
such as connecting multiple MRT islands across tunnels, but the need such as connecting multiple MRT Islands across tunnels, but the need
for the additional complexity has not been justified. for the additional complexity has not been justified.
12. Network Convergence and Preparing for the Next Failure 12. Network Convergence and Preparing for the Next Failure
After a failure, MRT detours ensure that packets reach their intended After a failure, MRT detours ensure that packets reach their intended
destination while the IGP has not reconverged onto the new topology. destination while the IGP has not reconverged onto the new topology.
As link-state updates reach the routers, the IGP process calculates As link-state updates reach the routers, the IGP process calculates
the new shortest paths. Two things need attention: micro-loop the new shortest paths. Two things need attention: micro-loop
prevention and MRT re-calculation. prevention and MRT recalculation.
12.1. Micro-loop prevention and MRTs 12.1. Micro-loop Prevention and MRTs
A micro-loop is a transient packet forwarding loop among two or more A micro-loop is a transient packet-forwarding loop among two or more
routers that can occur during convergence of IGP forwarding state. routers that can occur during convergence of IGP forwarding state.
[RFC5715] discusses several techniques for preventing micro-loops. [RFC5715] discusses several techniques for preventing micro-loops.
This section discusses how MRT-FRR relates to two of the micro-loop This section discusses how MRT-FRR relates to two of the micro-loop
prevention techniques discussed in [RFC5715], Nearside Tunneling and prevention techniques discussed in [RFC5715]: Nearside and Farside
Farside Tunneling. Tunneling.
In Nearside Tunneling, a router (PLR) adjacent to a failure perform In Nearside Tunneling, a router (PLR) adjacent to a failure performs
local repair and inform remote routers of the failure. The remote local repair and informs remote routers of the failure. The remote
routers initially tunnel affected traffic to the nearest PLR, using routers initially tunnel affected traffic to the nearest PLR, using
tunnels which are unaffected by the failure. Once the forwarding tunnels that are unaffected by the failure. Once the forwarding
state for normal shortest path routing has converged, the remote state for normal shortest path routing has converged, the remote
routers return the traffic to shortest path forwarding. MRT-FRR is routers return the traffic to shortest path forwarding. MRT-FRR is
relevant for Nearside Tunneling for the following reason. The relevant for Nearside Tunneling for the following reason. The
process of tunneling traffic to the PLRs and waiting a sufficient process of tunneling traffic to the PLRs and waiting a sufficient
amount of time for IGP forwarding state convergence with Nearside amount of time for IGP forwarding state convergence with Nearside
Tunneling means that traffic will generally be relying on the local Tunneling means that traffic will generally rely on the local repair
repair at the PLR for longer than it would in the absence of Nearside at the PLR for longer than it would in the absence of Nearside
Tunneling. Since MRT-FRR provides 100% coverage for single link and Tunneling. Since MRT-FRR provides 100% coverage for single link and
node failure, it may be an attractive option to provide the local node failure, it may be an attractive option to provide the local
repair paths when Nearside Tunneling is deployed. repair paths when Nearside Tunneling is deployed.
MRT-FRR is also relevant for the Farside Tunneling micro-loop MRT-FRR is also relevant for the Farside Tunneling micro-loop
prevention technique. In Farside Tunneling, remote routers tunnel prevention technique. In Farside Tunneling, remote routers tunnel
traffic affected by a failure to a node downstream of the failure traffic affected by a failure to a node downstream of the failure
with respect to traffic destination. This node can be viewed as with respect to traffic destination. This node can be viewed as
being on the farside of the failure with respect to the node being on the farside of the failure with respect to the node
initiating the tunnel. Note that the discussion of Farside Tunneling initiating the tunnel. Note that the discussion of Farside Tunneling
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mechanism used to reach the farside node must be unaffected by the mechanism used to reach the farside node must be unaffected by the
failure. The alternative forwarding paths created by MRT-FRR have failure. The alternative forwarding paths created by MRT-FRR have
the potential to be used to forward traffic from the remote routers the potential to be used to forward traffic from the remote routers
upstream of the failure all the way to the destination. In the event upstream of the failure all the way to the destination. In the event
of failure, either the MRT-Red or MRT-Blue path from the remote of failure, either the MRT-Red or MRT-Blue path from the remote
upstream router to the destination is guaranteed to avoid a link upstream router to the destination is guaranteed to avoid a link
failure or inferred node failure. The MRT forwarding paths are also failure or inferred node failure. The MRT forwarding paths are also
guaranteed to not be subject to micro-loops because they are locked guaranteed to not be subject to micro-loops because they are locked
to the topology before the failure. to the topology before the failure.
We note that the computations in [I-D.ietf-rtgwg-mrt-frr-algorithm] We note that the computations in [RFC7811] address the case of a PLR
address the case of a PLR adjacent to a failure determining which adjacent to a failure determining which choice of MRT-Red or MRT-Blue
choice of MRT-Red or MRT-Blue will avoid a failed link or node. More will avoid a failed link or node. More computation may be required
computation may be required for an arbitrary remote upstream router for an arbitrary remote upstream router to determine whether to
to determine whether to choose MRT-Red or MRT-Blue for a given choose MRT-Red or MRT-Blue for a given destination and failure.
destination and failure.
12.2. MRT Recalculation for the Default MRT Profile 12.2. MRT Recalculation for the Default MRT Profile
This section describes how the MRT recalculation SHOULD be performed This section describes how the MRT recalculation SHOULD be performed
for the Default MRT Profile. This is intended to support FRR for the Default MRT Profile. This is intended to support FRR
applications. Other approaches are possible, but they are not applications. Other approaches are possible, but they are not
specified in this document. specified in this document.
When a failure event happens, traffic is put by the PLRs onto the MRT When a failure event happens, traffic is put by the PLRs onto the MRT
topologies. After that, each router recomputes its shortest path topologies. After that, each router recomputes its SPT and moves
tree (SPT) and moves traffic over to that. Only after all the PLRs traffic over to that. Only after all the PLRs have switched to using
have switched to using their SPTs and traffic has drained from the their SPTs and traffic has drained from the MRT topologies should
MRT topologies should each router install the recomputed MRTs into each router install the recomputed MRTs into the FIBs.
the FIBs.
At each router, therefore, the sequence is as follows: At each router, therefore, the sequence is as follows:
1. Receive failure notification 1. Receive failure notification
2. Recompute SPT. 2. Recompute SPT.
3. Install the new SPT in the FIB. 3. Install the new SPT in the FIB.
4. If the network was stable before the failure occured, wait a 4. If the network was stable before the failure occurred, wait a
configured (or advertised) period for all routers to be using configured (or advertised) period for all routers to be using
their SPTs and traffic to drain from the MRTs. their SPTs and traffic to drain from the MRTs.
5. Recompute MRTs. 5. Recompute MRTs.
6. Install new MRTs in the FIB. 6. Install new MRTs in the FIB.
While the recomputed MRTs are not installed in the FIB, protection While the recomputed MRTs are not installed in the FIB, protection
coverage is lowered. Therefore, it is important to recalculate the coverage is lowered. Therefore, it is important to recalculate the
MRTs and install them quickly. MRTs and install them quickly.
New protocol extensions for advertising the time needed to recompute New protocol extensions for advertising the time needed to recompute
shortest path routes and install them in the FIB will be defined shortest path routes and install them in the FIB will be defined
elsewhere. elsewhere.
13. Implementation Status 13. Operational Considerations
[RFC Editor: please remove this section prior to publication.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC6982], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
Juniper Networks Implementation
o Organization responsible for the implementation: Juniper Networks
o Implementation name: MRT-FRR
o Implementation description: MRT-FRR using OSPF as the IGP has been
implemented and verified.
o The implementation's level of maturity: prototype
o Protocol coverage: This implementation of the MRT-FRR includes
Island identification, GADAG root selection, MRT Lowpoint
algorithm, augmentation of GADAG with additional links, and
calculation of MRT transit next-hops alternate next-hops based on
draft "draft-ietf-rtgwg-mrt-frr-algorithm-00". This
implementation also includes the M-bit in OSPF based on "draft-
atlas-ospf-mrt-01" as well as LDP MRT Capability based on "draft-
atlas-mpls-ldp-mrt-00".
o Licensing: proprietary
o Implementation experience: Implementation was useful for verifying
functionality and lack of gaps. It has also been useful for
improving aspects of the algorithm.
o Contact information: akatlas@juniper.net, shraddha@juniper.net,
kishoret@juniper.net
Huawei Technology Implementation
o Organization responsible for the implementation: Huawei Technology
Co., Ltd.
o Implementation name: MRT-FRR and IS-IS extensions for MRT.
o Implementation description: The MRT-FRR using IS-IS extensions for
MRT and LDP multi-topology have been implemented and verified.
o The implementation's level of maturity: prototype
o Protocol coverage: This implementation of the MRT algorithm
includes Island identification, GADAG root selection, MRT Lowpoint
algorithm, augmentation of GADAG with additional links, and
calculation of MRT transit next-hops alternate next-hops based on
draft "draft-enyedi-rtgwg-mrt-frr-algorithm-03". This
implementation also includes IS-IS extension for MRT based on
"draft-li-mrt-00".
o Licensing: proprietary
o Implementation experience: It is important produce a second
implementation to verify the algorithm is implemented correctly
without looping. It is important to verify the IS-IS extensions
work for MRT-FRR.
o Contact information: lizhenbin@huawei.com, eric.wu@huawei.com
14. Operational Considerations
The following aspects of MRT-FRR are useful to consider when The following aspects of MRT-FRR are useful to consider when
deploying the technology in different operational environments and deploying the technology in different operational environments and
network topologies. network topologies.
14.1. Verifying Forwarding on MRT Paths 13.1. Verifying Forwarding on MRT Paths
The forwarding paths created by MRT-FRR are not used by normal (non- The forwarding paths created by MRT-FRR are not used by normal (non-
FRR) traffic. They are only used to carry FRR traffic for a short FRR) traffic. They are only used to carry FRR traffic for a short
period of time after a failure has been detected. It is RECOMMENDED period of time after a failure has been detected. It is RECOMMENDED
that an operator proactively monitor the MRT forwarding paths in that an operator proactively monitor the MRT forwarding paths in
order to be certain that the paths will be able to carry FRR traffic order to be certain that the paths will be able to carry FRR traffic
when needed. Therefore, an implementation SHOULD provide an operator when needed. Therefore, an implementation SHOULD provide an operator
with the ability to test MRT paths with Operations, Administration, with the ability to test MRT paths with Operations, Administration,
and Maintenance (OAM) traffic. For example, when MRT paths are and Maintenance (OAM) traffic. For example, when MRT paths are
realized using LDP labels distributed for topology-scoped FECs, an realized using LDP labels distributed for topology-scoped FECs, an
implementation can use the MPLS ping and traceroute as defined in implementation can use the MPLS ping and traceroute as defined in
[RFC4379] and extended in [RFC7307] for topology-scoped FECs. [RFC4379] and extended in [RFC7307] for topology-scoped FECs.
14.2. Traffic Capacity on Backup Paths 13.2. Traffic Capacity on Backup Paths
During a fast-reroute event initiated by a PLR in response to a During a fast-reroute event initiated by a PLR in response to a
network failure, the flow of traffic in the network will generally network failure, the flow of traffic in the network will generally
not be identical to the flow of traffic after the IGP forwarding not be identical to the flow of traffic after the IGP forwarding
state has converged, taking the failure into account. Therefore, state has converged, taking the failure into account. Therefore,
even if a network has been engineered to have enough capacity on the even if a network has been engineered to have enough capacity on the
appropriate links to carry all traffic after the IGP has converged appropriate links to carry all traffic after the IGP has converged
after the failure, the network may still not have enough capacity on after the failure, the network may still not have enough capacity on
the appropriate links to carry the flow of traffic during a fast- the appropriate links to carry the flow of traffic during a fast-
reroute event. This can result in more traffic loss during the fast- reroute event. This can result in more traffic loss during the fast-
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alternate selection policy can be evaluated in the context of their alternate selection policy can be evaluated in the context of their
effect on fast-reroute traffic flow and available capacity, as well effect on fast-reroute traffic flow and available capacity, as well
as other deployment considerations. as other deployment considerations.
Note that future documents may define MRT profiles in addition to the Note that future documents may define MRT profiles in addition to the
default profile defined here. Different MRT profiles will generally default profile defined here. Different MRT profiles will generally
produce alternate paths with different properties. An implementation produce alternate paths with different properties. An implementation
may allow an operator to use different MRT profiles instead of or in may allow an operator to use different MRT profiles instead of or in
addition to the default profile. addition to the default profile.
14.3. MRT IP Tunnel Loopback Address Management 13.3. MRT IP Tunnel Loopback Address Management
As described in Section 6.1.2, if an implementation uses IP tunneling As described in Section 6.1.2, if an implementation uses IP tunneling
as the mechanism to realize MRT forwarding paths, each node must as the mechanism to realize MRT forwarding paths, each node must
advertise an MRT-Red and an MRT-Blue loopback address. These IP advertise an MRT-Red and an MRT-Blue loopback address. These IP
addresses must be unique within the routing domain to the extent that addresses must be unique within the routing domain to the extent that
they do not overlap with each other or with any other routing table they do not overlap with each other or with any other routing table
entries. It is expected that operators will use existing tools and entries. It is expected that operators will use existing tools and
processes for managing infrastructure IP addresses to manage these processes for managing infrastructure IP addresses to manage these
additional MRT-related loopback addresses. additional MRT-related loopback addresses.
14.4. MRT-FRR in a Network with Degraded Connectivity 13.4. MRT-FRR in a Network with Degraded Connectivity
Ideally, routers is a service provider network using MRT-FRR will be Ideally, routers in a service provider network using MRT-FRR will be
initially deployed in a 2-connected topology, allowing MRT-FRR to initially deployed in a 2-connected topology, allowing MRT-FRR to
find completely diverse paths to all destinations. However, a find completely diverse paths to all destinations. However, a
network can differ from an ideal 2-connected topology for many network can differ from an ideal 2-connected topology for many
possible reasons, including network failures and planned maintenance possible reasons, including network failures and planned maintenance
events. events.
MRT-FRR is designed to continue to function properly when network MRT-FRR is designed to continue to function properly when network
connectivity is degraded. When a network contains cut-vertices or connectivity is degraded. When a network contains cut-vertices or
cut-links dividing the network into different 2-connected blocks, cut-links dividing the network into different 2-connected blocks,
MRT-FRR will continue to provide completely diverse paths for MRT-FRR will continue to provide completely diverse paths for
skipping to change at page 38, line 41 skipping to change at page 37, line 5
FRR will be link and node diverse within each block, and the paths FRR will be link and node diverse within each block, and the paths
will only share links and nodes that are cut-links or cut-vertices in will only share links and nodes that are cut-links or cut-vertices in
the topology. the topology.
If a network becomes partitioned with one set of routers having no If a network becomes partitioned with one set of routers having no
connectivity to another set of routers, MRT-FRR will function connectivity to another set of routers, MRT-FRR will function
independently in each set of connected routers, providing redundant independently in each set of connected routers, providing redundant
paths to destinations in same set of connected routers as a given paths to destinations in same set of connected routers as a given
PLR. PLR.
14.5. Partial Deployment of MRT-FRR in a Network 13.5. Partial Deployment of MRT-FRR in a Network
A network operator may choose to deploy MRT-FRR only on a subset of A network operator may choose to deploy MRT-FRR only on a subset of
routers in an IGP area. MRT-FRR is designed to accommodate this routers in an IGP area. MRT-FRR is designed to accommodate this
partial deployment scenario. Only routers that advertise support for partial deployment scenario. Only routers that advertise support for
a given MRT profile will be included in a given MRT Island. For a a given MRT profile will be included in a given MRT Island. For a
PLR within the MRT Island, MRT-FRR will create redundant forwarding PLR within the MRT Island, MRT-FRR will create redundant forwarding
paths to all destinations with the MRT Island using maximally paths to all destinations with the MRT Island using maximally
redundant trees all the way to those destinations. For destinations redundant trees all the way to those destinations. For destinations
outside of the MRT Island, MRT-FRR creates paths to the destination outside of the MRT Island, MRT-FRR creates paths to the destination
which use forwarding state created by MRT-FRR within the MRT Island that use forwarding state created by MRT-FRR within the MRT Island
and shortest path forwarding state outside of the MRT Island. The and shortest path forwarding state outside of the MRT Island. The
paths created by MRT-FRR to non-Island destinations are guaranteed to paths created by MRT-FRR to non-Island destinations are guaranteed to
be diverse within the MRT Island (if topologically possible). be diverse within the MRT Island (if topologically possible).
However, the part of the paths outside of the MRT Island may not be However, the part of the paths outside of the MRT Island may not be
diverse. diverse.
15. Acknowledgements 14. IANA Considerations
The authors would like to thank Mike Shand for his valuable review
and contributions.
The authors would like to thank Joel Halpern, Hannes Gredler, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and
Alvaro Retana for their suggestions and review.
16. IANA Considerations
IANA is requested to create a registry entitled "MRT Profile IANA has created the "MRT Profile Identifier Registry". The range is
Identifier Registry". The range is 0 to 255. The Default MRT 0 to 255. The Default MRT Profile defined in this document has value
Profile defined in this document has value 0. Values 1-200 are 0. Values 1-200 are allocated by Standards Action. Values 201-220
allocated by Standards Action. Values 201-220 are for Experimental are for Experimental Use. Values 221-254 are for Private Use. Value
Use. Values 221-254 are for Private Use. Value 255 is reserved for 255 is reserved for future registry extension. (The allocation and
future registry extension. (The allocation and use policies are use policies are described in [RFC5226].)
described in [RFC5226].)
The initial registry is shown below. The initial registry is shown below.
Value Description Reference Value Description Reference
------- ---------------------------------------- ------------ ------- ---------------------------------------- ------------
0 Default MRT Profile [This draft] 0 Default MRT Profile RFC 7812
1-200 Unassigned 1-200 Unassigned
201-220 Experimental Use 201-220 Experimental Use
221-254 Private Use 221-254 Private Use
255 Reserved (for future registry extension) 255 Reserved (for future registry extension)
The MRT Profile Identifier Registry is a new registry in the IANA The "MRT Profile Identifier Registry" is a new registry in the IANA
Matrix. Following existing conventions, http://www.iana.org/ Matrix. Following existing conventions, http://www.iana.org/
protocols should display a new header entitled "Maximally Redundant protocols displays a new header: "Maximally Redundant Tree (MRT)
Tree (MRT) Parameters". Under that header, there should be an entry Parameters". Under that header, there is an entry for "MRT Profile
for "MRT Profile Identifier Registry" with a link to the registry Identifier Registry", which links to the registry itself at
itself at http://www.iana.org/assignments/mrt-parameters/mrt- http://www.iana.org/assignments/mrt-parameters.
parameters.xhtml#mrt-profile-registry.
17. Security Considerations 15. Security Considerations
In general, MRT forwarding paths do not follow shortest paths. The In general, MRT forwarding paths do not follow shortest paths. The
transit forwarding state corresponding to the MRT paths is created transit forwarding state corresponding to the MRT paths is created
during normal operations (before a failure occurs). Therefore, a during normal operations (before a failure occurs). Therefore, a
malicious packet with an appropriate header injected into the network malicious packet with an appropriate header injected into the network
from a compromised location would be forwarded to a destination along from a compromised location would be forwarded to a destination along
a non-shortest path. When this technology is deployed, a network a non-shortest path. When this technology is deployed, a network
security design should not rely on assumptions about potentially security design should not rely on assumptions about potentially
malicious traffic only following shortest paths. malicious traffic only following shortest paths.
skipping to change at page 40, line 31 skipping to change at page 38, line 31
the protocol extensions used to advertise this information, we the protocol extensions used to advertise this information, we
discuss security considerations related to the information itself. discuss security considerations related to the information itself.
Injecting false MRT-related information could be used to direct some Injecting false MRT-related information could be used to direct some
MRT backup paths over compromised transmission links. Combined with MRT backup paths over compromised transmission links. Combined with
the ability to generate network failures, this could be used to send the ability to generate network failures, this could be used to send
traffic over compromised transmission links during a fast-reroute traffic over compromised transmission links during a fast-reroute
event. In order to prevent this potential exploit, a receiving event. In order to prevent this potential exploit, a receiving
router needs to be able to authenticate MRT-related information that router needs to be able to authenticate MRT-related information that
claims to have been advertised by another router. claims to have been advertised by another router.
18. Contributors 16. References
Robert Kebler
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: rkebler@juniper.net
Andras Csaszar
Ericsson
Konyves Kalman krt 11
Budapest 1097
Hungary
Email: Andras.Csaszar@ericsson.com
Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
USA
Email: jeff.tantsura@ericsson.com
Russ White
VCE
Email: russw@riw.us
19. References
19.1. Normative References
[I-D.ietf-rtgwg-mrt-frr-algorithm] 16.1. Normative References
Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
Gopalan, "Algorithms for computing Maximally Redundant
Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr-
algorithm-06 (work in progress), October 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008, DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>. <http://www.rfc-editor.org/info/rfc5226>.
[RFC7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D. [RFC7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi-Topology", RFC 7307, King, "LDP Extensions for Multi-Topology", RFC 7307,
DOI 10.17487/RFC7307, July 2014, DOI 10.17487/RFC7307, July 2014,
<http://www.rfc-editor.org/info/rfc7307>. <http://www.rfc-editor.org/info/rfc7307>.
19.2. Informative References [RFC7811] Enyedi, G., Ed., Csaszar, A., Atlas, A., Ed., Bowers, C.,
and A. Gopalan, "An Algorithm for Computing IP/LDP Fast
Reroute Using Maximally Redundant Trees (MRT-FRR)",
RFC 7811, DOI 10.17487/RFC7811, June 2016,
<http://www.rfc-editor.org/info/rfc7811>.
16.2. Informative References
[EnyediThesis] [EnyediThesis]
Enyedi, G., "Novel Algorithms for IP Fast Reroute", Enyedi, G., "Novel Algorithms for IP Fast Reroute",
Department of Telecommunications and Media Informatics, Department of Telecommunications and Media Informatics,
Budapest University of Technology and Economics Ph.D. Budapest University of Technology and Economics Ph.D.
Thesis, February 2011, Thesis, February 2011,
<http://timon.tmit.bme.hu/theses/thesis_book.pdf>. <https://repozitorium.omikk.bme.hu/bitstream/
handle/10890/1040/ertekezes.pdf>.
[I-D.atlas-rtgwg-mrt-mc-arch] [LDP-MRT] Atlas, A., Tiruveedhula, K., Bowers, C., Tantsura, J., and
Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G. IJ. Wijnands, "LDP Extensions to Support Maximally
Envedi, "An Architecture for Multicast Protection Using Redundant Trees", Work in Progress, draft-ietf-mpls-ldp-
Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc- mrt-03, May 2016.
arch-02 (work in progress), July 2013.
[MRT-ARCH]
Atlas, A., Kebler, R., Wijnands, IJ., Csaszar, A., and G.
Enyedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees", Work in Progress, draft-atlas-
rtgwg-mrt-mc-arch-02, July 2013.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998, DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>. <http://www.rfc-editor.org/info/rfc2328>.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379, Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006, DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>. <http://www.rfc-editor.org/info/rfc4379>.
skipping to change at page 43, line 24 skipping to change at page 40, line 32
Francois, P., and O. Bonaventure, "Framework for Loop-Free Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
2013, <http://www.rfc-editor.org/info/rfc6976>. 2013, <http://www.rfc-editor.org/info/rfc6976>.
[RFC6981] Bryant, S., Previdi, S., and M. Shand, "A Framework for IP [RFC6981] Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981, and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
DOI 10.17487/RFC6981, August 2013, DOI 10.17487/RFC6981, August 2013,
<http://www.rfc-editor.org/info/rfc6981>. <http://www.rfc-editor.org/info/rfc6981>.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
<http://www.rfc-editor.org/info/rfc6982>.
[RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D. [RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987, McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC6987, September 2013, DOI 10.17487/RFC6987, September 2013,
<http://www.rfc-editor.org/info/rfc6987>. <http://www.rfc-editor.org/info/rfc6987>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015, RFC 7490, DOI 10.17487/RFC7490, April 2015,
<http://www.rfc-editor.org/info/rfc7490>. <http://www.rfc-editor.org/info/rfc7490>.
Appendix A. Inter-level Forwarding Behavior for IS-IS Appendix A. Inter-level Forwarding Behavior for IS-IS
In the description below, we use the terms "Level-1-only interface", In the description below, we use the terms "Level-1-only interface",
"Level-2-only interface", and "Level-1-and-Level-2 interface" to mean "Level-2-only interface", and "Level-1-and-Level-2 interface" to mean
in interface which has formed only a Level-1 adjacency, only a an interface that has formed only a Level-1 adjacency, only a Level-2
Level-2 adjacency, or both Level-1 and Level-2 adjacencies. Note adjacency, or both Level-1 and Level-2 adjacencies. Note that IS-IS
that IS-IS also defines the concept of areas. A router is configured also defines the concept of areas. A router is configured with an
with an IS-IS area identifier, and a given router may be configured IS-IS area identifier, and a given router may be configured with
with multiple IS-IS area identifiers. For an IS-IS Level-1 adjacency multiple IS-IS area identifiers. For an IS-IS Level-1 adjacency to
to form between two routers, at least one IS-IS area identifier must form between two routers, at least one IS-IS area identifier must
match. IS-IS Level-2 adjacencies to not require any area identifiers match. IS-IS Level-2 adjacencies do not require any area identifiers
to match. The behavior described below does not explicitly refer to to match. The behavior described below does not explicitly refer to
IS-IS area identifiers. However, IS-IS area identifiers will IS-IS area identifiers. However, IS-IS area identifiers will
indirectly affect the behavior by affecting the formation of Level-1 indirectly affect the behavior by affecting the formation of Level-1
adjacencies. adjacencies.
First consider a packet destined to Z on MRT-Red or MRT-Blue received First, consider a packet destined to Z on MRT-Red or MRT-Blue
on a Level-1-only interface. If the best shortest path route to Z received on a Level-1-only interface. If the best shortest path
was learned from a Level-1 advertisement, then the packet should route to Z was learned from a Level-1 advertisement, then the packet
continue to be forwarded along MRT-Red or MRT-Blue. If instead the should continue to be forwarded along MRT-Red or MRT-Blue. If,
best route was learned from a Level-2 advertisement, then the packet instead, the best route was learned from a Level-2 advertisement,
should be removed from MRT-Red or MRT-Blue and forwarded on the then the packet should be removed from MRT-Red or MRT-Blue and
shortest-path default forwarding topology. forwarded on the shortest-path default forwarding topology.
Now consider a packet destined to Z on MRT-Red or MRT-Blue received Now consider a packet destined to Z on MRT-Red or MRT-Blue received
on a Level-2-only interface. If the best route to Z was learned from on a Level-2-only interface. If the best route to Z was learned from
a Level-2 advertisement, then the packet should continue to be a Level-2 advertisement, then the packet should continue to be
forwarded along MRT-Red or MRT-Blue. If instead the best route was forwarded along MRT-Red or MRT-Blue. If, instead, the best route was
learned from a Level-1 advertisement, then the packet should be learned from a Level-1 advertisement, then the packet should be
removed from MRT-Red or MRT-Blue and forwarded on the shortest-path removed from MRT-Red or MRT-Blue and forwarded on the shortest-path
default forwarding topology. default forwarding topology.
Finally, consider a packet destined to Z on MRT-Red or MRT-Blue Finally, consider a packet destined to Z on MRT-Red or MRT-Blue
received on a Level-1-and-Level-2 interface. This packet should received on a Level-1-and-Level-2 interface. This packet should
continue to be forwarded along MRT-Red or MRT-Blue, regardless of continue to be forwarded along MRT-Red or MRT-Blue, regardless of
which level the route was learned from. which level the route was learned from.
An implementation may simplify the decision-making process above by An implementation may simplify the decision-making process above by
using the interface of the next-hop for the route to Z to determine using the interface of the next hop for the route to Z to determine
the level that the best route to Z was learned from. If the next-hop the level from which the best route to Z was learned. If the next
points out a Level-1-only interface, then the route was learned from hop points out a Level-1-only interface, then the route was learned
a Level-1 advertisement. If the next-hop points out a Level-2-only from a Level-1 advertisement. If the next hop points out a Level-
interface, then the route was learned from a Level-2 advertisement. 2-only interface, then the route was learned from a Level-2
A next-hop that points out a Level-1-and-Level-2 interface does not advertisement. A next hop that points out a Level-1-and-Level-2
provide enough information to determine the source of the best route. interface does not provide enough information to determine the source
With this simplification, an implementation would need to continue of the best route. With this simplification, an implementation would
forwarding along MRT-Red or MRT-Blue when the next-hop points out a need to continue forwarding along MRT-Red or MRT-Blue when the next-
Level-1-and-Level-2 interface. Therefore, a packet on MRT-Red or hop points out a Level-1-and-Level-2 interface. Therefore, a packet
MRT-Blue going from Level-1 to Level-2 (or vice versa) that traverses on MRT-Red or MRT-Blue going from Level-1 to Level-2 (or vice versa)
a Level-1-and-Level-2 interface in the process will remain on MRT-Red that traverses a Level-1-and-Level-2 interface in the process will
or MRT-Blue. This simplification may not always produce the optimal remain on MRT-Red or MRT-Blue. This simplification may not always
forwarding behavior, but it does not introduce interoperability produce the optimal forwarding behavior, but it does not introduce
problems. The packet will stay on an MRT backup path longer than interoperability problems. The packet will stay on an MRT backup
necessary, but it will still reach its destination. path longer than necessary, but it will still reach its destination.
Appendix B. General Issues with Area Abstraction Appendix B. General Issues with Area Abstraction
When a multi-homed prefix is connected in two different areas, it may When a multihomed prefix is connected in two different areas, it may
be impractical to protect them without adding the complexity of be impractical to protect them without adding the complexity of
explicit tunneling. This is also a problem for LFA and Remote-LFA. explicit tunneling. This is also a problem for LFA and Remote-LFA.
50 50
|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0: |----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:
| | ABR 1, ABR 2, C, D | | ABR 1, ABR 2, C, D
| | | |
| | Area 20: A, ASBR X | | Area 20: A, ASBR X
| | | |
p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y
5 p is a Type 1 AS-external 5 p is a Type 1 AS-external
Figure 4: AS external prefixes in different areas Figure 4: AS External Prefixes in Different Areas
Consider the network in Figure 4 and assume there is a richer Consider the network in Figure 4 and assume there is a richer
connective topology that isn't shown, where the same prefix is connective topology that isn't shown, where the same prefix is
announced by ASBR X and ASBR Y which are in different non-backbone announced by ASBR X and ASBR Y, which are in different non-backbone
areas. If the link from A to ASBR X fails, then an MRT alternate areas. If the link from A to ASBR X fails, then an MRT alternate
could forward the packet to ABR 1 and ABR 1 could forward it to D, could forward the packet to ABR 1 and ABR 1 could forward it to D,
but then D would find the shortest route is back via ABR 1 to Area but then D would find the shortest route is back via ABR 1 to Area
20. This problem occurs because the routers, including the ABR, in 20. This problem occurs because the routers, including the ABR, in
one area are not yet aware of the failure in a different area. one area are not yet aware of the failure in a different area.
The only way to get it from A to ASBR Y is to explicitly tunnel it to The only way to get it from A to ASBR Y is to explicitly tunnel it to
ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels
are known, then explicit tunneling MAY be used as long as the are known, then explicit tunneling MAY be used as long as the
shortest-path of the tunnel avoids the failure point. In that case, shortest path of the tunnel avoids the failure point. In that case,
A must determine that it should use an explicit tunnel instead of an A must determine that it should use an explicit tunnel instead of an
MRT alternate. MRT alternate.
Acknowledgements
The authors would like to thank Mike Shand for his valuable review
and contributions.
The authors would like to thank Joel Halpern, Hannes Gredler, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and
Alvaro Retana for their suggestions and review.
Contributors
Robert Kebler
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States
Email: rkebler@juniper.net
Andras Csaszar
Ericsson
Konyves Kalman krt 11
Budapest 1097
Hungary
Email: Andras.Csaszar@ericsson.com
Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
United States
Email: jeff.tantsura@ericsson.com
Russ White
VCE
Email: russw@riw.us
Authors' Addresses Authors' Addresses
Alia Atlas Alia Atlas
Juniper Networks Juniper Networks
10 Technology Park Drive 10 Technology Park Drive
Westford, MA 01886 Westford, MA 01886
USA United States
Email: akatlas@juniper.net Email: akatlas@juniper.net
Chris Bowers Chris Bowers
Juniper Networks Juniper Networks
1194 N. Mathilda Ave. 1194 N. Mathilda Ave.
Sunnyvale, CA 94089 Sunnyvale, CA 94089
USA United States
Email: cbowers@juniper.net Email: cbowers@juniper.net
Gabor Sandor Enyedi Gabor Sandor Enyedi
Ericsson Ericsson
Konyves Kalman krt 11. Konyves Kalman krt 11.
Budapest 1097 Budapest 1097
Hungary Hungary
Email: Gabor.Sandor.Enyedi@ericsson.com Email: Gabor.Sandor.Enyedi@ericsson.com
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