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Versions: (draft-atlas-rtgwg-mrt-frr-architecture) 00 01 02 03 04 05 06 07 08 09 10 RFC 7812

Routing Area Working Group                                      A. Atlas
Internet-Draft                                                 C. Bowers
Intended status: Standards Track                        Juniper Networks
Expires: July 13, 2016                                         G. Enyedi
                                                                Ericsson
                                                        January 10, 2016


An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
                draft-ietf-rtgwg-mrt-frr-architecture-09

Abstract

   This document defines the architecture for IP/LDP Fast-Reroute using
   Maximally Redundant Trees (MRT-FRR).  MRT-FRR is a technology that
   gives link-protection and node-protection with 100% coverage in any
   network topology that is still connected after the failure.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 13, 2016.

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   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Importance of 100% Coverage . . . . . . . . . . . . . . .   8
     1.2.  Partial Deployment and Backwards Compatibility  . . . . .   9
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   9
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . .  11
   5.  Maximally Redundant Trees (MRT) and Fast-Reroute  . . . . . .  12
   6.  Unicast Forwarding with MRT Fast-Reroute  . . . . . . . . . .  13
     6.1.  MRT Forwarding Mechanisms . . . . . . . . . . . . . . . .  14
       6.1.1.  MRT LDP labels  . . . . . . . . . . . . . . . . . . .  14
         6.1.1.1.  Topology-scoped FEC encoded using a single label
                   (Option 1A) . . . . . . . . . . . . . . . . . . .  14
         6.1.1.2.  Topology and FEC encoded using a two label stack
                   (Option 1B) . . . . . . . . . . . . . . . . . . .  15
         6.1.1.3.  Compatibility of Option 1A and 1B . . . . . . . .  15
         6.1.1.4.  Mandatory support for MRT LDP Label option 1A . .  15
       6.1.2.  MRT IP tunnels (Options 2A and 2B)  . . . . . . . . .  16
     6.2.  Forwarding LDP Unicast Traffic over MRT Paths . . . . . .  16
       6.2.1.  Forwarding LDP traffic using MRT LDP Labels (Option
               1A) . . . . . . . . . . . . . . . . . . . . . . . . .  17
       6.2.2.  Forwarding LDP traffic using MRT LDP Labels (Option
               1B) . . . . . . . . . . . . . . . . . . . . . . . . .  17
       6.2.3.  Other considerations for forwarding LDP traffic using
               MRT LDP Labels  . . . . . . . . . . . . . . . . . . .  17
     6.3.  Forwarding IP Unicast Traffic over MRT Paths  . . . . . .  18
       6.3.1.  Tunneling IP traffic using MRT LDP Labels . . . . . .  18
         6.3.1.1.  Tunneling IP traffic using MRT LDP Labels (Option
                   1A) . . . . . . . . . . . . . . . . . . . . . . .  18
         6.3.1.2.  Tunneling IP traffic using MRT LDP Labels (Option
                   1B) . . . . . . . . . . . . . . . . . . . . . . .  19
       6.3.2.  Tunneling IP traffic using MRT IP Tunnels . . . . . .  19
       6.3.3.  Required support  . . . . . . . . . . . . . . . . . .  19
   7.  MRT Island Formation  . . . . . . . . . . . . . . . . . . . .  19
     7.1.  IGP Area or Level . . . . . . . . . . . . . . . . . . . .  20
     7.2.  Support for a specific MRT profile  . . . . . . . . . . .  20
     7.3.  Excluding additional routers and interfaces from the MRT
           Island  . . . . . . . . . . . . . . . . . . . . . . . . .  21
       7.3.1.  Existing IGP exclusion mechanisms . . . . . . . . . .  21
       7.3.2.  MRT-specific exclusion mechanism  . . . . . . . . . .  22
     7.4.  Connectivity  . . . . . . . . . . . . . . . . . . . . . .  22
     7.5.  Algorithm for MRT Island Identification . . . . . . . . .  22
   8.  MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  MRT Profile Options . . . . . . . . . . . . . . . . . . .  22



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     8.2.  Router-specific MRT paramaters  . . . . . . . . . . . . .  24
     8.3.  Default MRT profile . . . . . . . . . . . . . . . . . . .  24
   9.  LDP signaling extensions and considerations . . . . . . . . .  25
   10. Inter-area Forwarding Behavior  . . . . . . . . . . . . . . .  25
     10.1.  ABR Forwarding Behavior with MRT LDP Label Option 1A . .  26
       10.1.1.  Motivation for Creating the Rainbow-FEC  . . . . . .  27
     10.2.  ABR Forwarding Behavior with IP Tunneling (option 2) . .  27
     10.3.  ABR Forwarding Behavior with LDP Label option 1B . . . .  28
   11. Prefixes Multiply Attached to the MRT Island  . . . . . . . .  29
     11.1.  Protecting Multi-Homed Prefixes using Tunnel Endpoint
            Selection  . . . . . . . . . . . . . . . . . . . . . . .  31
     11.2.  Protecting Multi-Homed Prefixes using Named Proxy-Nodes   32
     11.3.  MRT Alternates for Destinations Outside the MRT Island .  34
   12. Network Convergence and Preparing for the Next Failure  . . .  34
     12.1.  Micro-loop prevention and MRTs . . . . . . . . . . . . .  35
     12.2.  MRT Recalculation for the Default MRT Profile  . . . . .  36
   13. Implementation Status . . . . . . . . . . . . . . . . . . . .  37
   14. Operational Considerations  . . . . . . . . . . . . . . . . .  38
     14.1.  Verifying Forwarding on MRT Paths  . . . . . . . . . . .  38
     14.2.  Traffic Capacity on Backup Paths . . . . . . . . . . . .  39
     14.3.  MRT IP Tunnel Loopback Address Management  . . . . . . .  40
     14.4.  MRT-FRR in a Network with Degraded Connectivity  . . . .  41
     14.5.  Partial Deployment of MRT-FRR in a Network . . . . . . .  41
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  41
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
   17. Security Considerations . . . . . . . . . . . . . . . . . . .  42
   18. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  42
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  43
     19.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  General Issues with Area Abstraction . . . . . . . .  46
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

1.  Introduction

   This document describes a solution for IP/LDP fast-reroute [RFC5714].
   MRT-FRR creates two alternate trees separate from the primary next-
   hop forwarding used during stable operation.  These two trees are
   maximally diverse from each other, providing link and node protection
   for 100% of paths and failures as long as the failure does not cut
   the network into multiple pieces.  This document defines the
   architecture for IP/LDP fast-reroute with MRT.

   [I-D.ietf-rtgwg-mrt-frr-algorithm] describes how to compute maximally
   redundant trees using a specific algorithm, the MRT Lowpoint
   algorithm.  The MRT Lowpoint algorithm is used by a router that
   supports the Default MRT Profile, as specified in this document.




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   IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse
   forwarding topologies to provide alternates.  A primary next-hop
   should be on only one of the diverse forwarding topologies; thus, the
   other can be used to provide an alternate.  Once traffic has been
   moved to one of the MRTs by one point of local repair (PLR), that
   traffic is not subject to further repair actions by another PLR, even
   in the event of multiple simultaneous failures.  Therefore, traffic
   repaired by MRT-FRR will not loop between different PLRs responding
   to different simultaneous failures.

   While MRT provides 100% protection for a single link or node failure,
   it may not protect traffic in the event of multiple simultaneous
   failures, nor does take into account Shared Risk Link Groups (SRLGs).
   Also, while the MRT Lowpoint algorithm is computationally efficient,
   it is also new.  In order for MRT-FRR to function properly, all of
   the other nodes in the network that support MRT must correctly
   compute next-hops based on the same algorithm, and install the
   corresponding forwarding state.  This is in contrast to other FRR
   methods where the calculation of backup paths generally involves
   repeated application of the simpler and widely-deployed SPF
   algorithm, and backup paths themselves re-use the forwarding state
   used for shortest path forwarding of normal traffic.  Section 14
   provides operational guidance related to verification of MRT
   forwarding paths.

   In addition to supporting IP and LDP unicast fast-reroute, the
   diverse forwarding topologies and guarantee of 100% coverage permit
   fast-reroute technology to be applied to multicast traffic as
   described in [I-D.atlas-rtgwg-mrt-mc-arch].  However, the current
   document does not address the multicast applications of MRTs.

   Figure 1 compares different methods of providing FRR for IP and LDP
   traffic, illustrating some of the trade-offs between the different
   approaches.  For several methods, the methods are separated into
   link-protecting (LP) and node-protecting (NP) variants.  The first
   column indicates whether the method provides 100% protection coverage
   (when topologically feasible).  The second column indicates whether
   or not traffic traversing alternate path can loop when multiple
   failures occur.

   The third column gives an estimate of the amount of computation
   required at each node to support the FRR method, in addition to the
   usual SPF computation rooted at the computing node itself.  This
   metric of comparison is important for implementations that seek to
   quickly recompute repair paths following their initial use after a
   topology change, without reliance on an external system, in order to
   minimize the risk of a new failure occurring before the new repair
   paths are in place.



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   The fourth column indicates the maximum number of additional labels
   that need to be applied to packets to support the FRR method, while
   the fifth column gives the size of the MPLS label table needed to
   support the FRR method.  These two metrics may be useful for
   evaluating requirements placed on hardware to support the different
   FRR methods.

   The last column indicates the additional requirements placed on the
   control plane by the FRR method, beyond what is required for IGP
   shortest path forwarding with LDP.









































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 +---------+-----+------+-------------+-------+-------------+----------+
 | Method  |100% | Alt. | Additional  | Max.  | Label table | Control  |
 |         |prot.| can  | computation | addit.|    size     | plane    |
 |         |cov. | loop?| at each     | labels|(relative to | reqs.    |
 |         |     |      | node        |       | SPF labels) |          |
 +---------+-----+------+-------------+-------+-------------+----------+
 | MRT-FRR | Yes | No   | equivalent  | 0(LDP)|     3x      | MRT-     |
 | LP and  |     |      | of less     | 1(IP) |             | specific |
 | NP      |     |      | than        |       |             | protocol |
 |         |     |      | 3 SPFs      |       |             | extens.  |
 +---------+-----+------+-------------+-------+-------------+----------+
 | LFA LP  | No  | Yes  |  SPF per    |   0   |     1x      | None     |
 | and NP  |     |      |  neighbor   |       |             |          |
 +---------+-----+------+-------------+-------+-------------+----------+
 | Remote  | No  | Yes  |  SPF per    |   1   |     1x      | T-LDP    |
 | LFA LP  |     |      |  neighbor   |       |             | session  |
 |         |     |      |             |       |             | for each |
 |         |     |      |             |       |             | selected |
 |         |     |      |             |       |             | PQ node  |
 +---------+-----+------+-------------+-------+-------------+----------+
 | Remote  | No  | Yes  | SPF per nbr |   1   |     1x      | T-LDP    |
 | LFA NP  |     |      | and SPF per |       |             | session  |
 |         |     |      | per PQ node |       |             | for each |
 |         |     |      | evaluated   |       |             | selected |
 |         |     |      |             |       |             | PQ node  |
 +---------+-----+------+-------------+-------+-------------+----------+
 | Not-Via | Yes | No   | SPF per     |   1   |  (average   | T-LDP    |
 | LP and  |     |      | link and    |       |  number of  | session  |
 | NP      |     |      | per node    |       |  neighbors) | for each |
 |         |     |      | in topology |       |      x      | nbr's nbr|
 +---------+-----+------+-------------+-------+-------------+----------+
 | TI-LFA  | Yes | Yes  | SPF per     |   2   |     1x      | uses     |
 | LP with |     |      | neighbor    |       |             | SPRING   |
 | symm.   |     |      |             |       |             | for      |
 | metrics |     |      |             |       |             | label    |
 |         |     |      |             |       |             | dist.    |
 +---------+-----+------+-------------+-------+-------------+----------+
 | TI-LFA  | Yes | Yes  | # of SPFs   |depth  |     1x      | uses     |
 | NP or   |     |      | dependent   |depend-|             | SPRING   |
 | LP with |     |      | on topology |ent on |             | for      |
 | asymm.  |     |      |             |topo.  |             | label    |
 | metrics |     |      |             |       |             | dist.    |
 +---------+-----+------+-------------+-------+-------------+----------+

                Figure 1: Comparison of IP/LDP FRR Methods

   MRT:   MRT provides 100% coverage for link and node protection, and
      traffic on the alternate paths will not loop.  The computation



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      required on each node is equivalent to less than 3 additional SPFs
      in terms of computational complexity.  For IP traffic, MRT
      requires one additional label, while for LDP traffic, no
      additional labels are needed.  However, the size of the MPLS label
      table is three times what would normally be required for shortest
      path LDP forwarding, due to the presence of additional red and
      blue labels for each FEC.  MRT requires protocol extensions in
      order to allow a node to advertise support for MRT as well as a
      value for the GADAG Root Selection Priority.  It also requires
      support for multi-topology LDP extensions.

   Loop-Free Alternates (LFA):   LFAs [RFC5286] provide limited
      topology-dependent coverage for link and node protection.
      Restrictions on choice of alternates can be relaxed to improve
      coverage, but this can cause forwarding loops if a worse failure
      is experienced than protected against.  [RFC6571] discusses the
      applicability of LFA to different topologies with a focus on
      common PoP architectures.  The computation required is one SPF per
      neighbor.  LFA imposes no additional labels imposed, has no impact
      on the label table size, and has no additional control plane
      requirements.

   Remote LFA:   Remote LFA [RFC7490] improves coverage over LFA for
      both link and node protection, but it does not guarantee 100%
      coverage.  The alternates can also loop with worse than expected
      failures.  Computation for link protection is one SPF per
      neighbor, while computation for node protection requires an
      additional SPF per PQ node [I-D.ietf-rtgwg-rlfa-node-protection].
      Remote LFA can impose up to one additional label on the packet,
      but does not increase the size of the label table.  It requires a
      T-LDP session for each selected PQ node.

   Not-Via:   Not-Via [RFC6981] provides 100% coverage for link and node
      failures and does not have potential looping among alternates.
      The computation is high with an SPF per potential failure point
      (all links and nodes in the topology).  When implemented with LDP,
      Not-Via adds one additional label to a tunnelled packet.  It
      significantly increases the size of the label table, multiplying
      it by roughly the average number of neighbors.  Not-Via also
      requires targeted LDP sessions to each neighbor's neighbor.

   TI-LFA:   Topology Independent Loop-free Alternate Fast Re-route (TI-
      LFA) [I-D.francois-rtgwg-segment-routing-ti-lfa] aims to provide
      link and node protection of node and adjacency segments within the
      Segment Routing (SR) framework.  It guarantees complete coverage.
      The TI-LFA computation for link-protection is fairly
      straightforward, while the computation for node-protection is more
      complex.  For link-protection with symmetric link costs, TI-LFA



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      can provide complete coverage by pushing up to two additional
      labels.  For node protection on arbitrary topologies, the label
      stack size can grow significantly based on repair path.  Note that
      TI-LFA requires shortest path forwarding based on SR Node-SIDs, as
      opposed to LDP labels, in order to construct label stacks for
      backups paths without relying on a large number of targeted LDP
      sessions to learn remote FEC-label bindings.  It also requires the
      use of Adj-SIDs to achieve 100% coverage.

1.1.  Importance of 100% Coverage

   Fast-reroute is based upon the single failure assumption - that the
   time between single failures is long enough for a network to
   reconverge and start forwarding on the new shortest paths.  That does
   not imply that the network will only experience one failure or
   change.

   It is straightforward to analyze a particular network topology for
   coverage.  However, a real network does not always have the same
   topology.  For instance, maintenance events will take links or nodes
   out of use.  Simply costing out a link can have a significant effect
   on what LFAs are available.  Similarly, after a single failure has
   happened, the topology is changed and its associated coverage.
   Finally, many networks have new routers or links added and removed;
   each of those changes can have an effect on the coverage for
   topology-sensitive methods such as LFA and Remote LFA.  If fast-
   reroute is important for the network services provided, then a method
   that guarantees 100% coverage is important to accomodate natural
   network topology changes.

   Asymmetric link costs are also a common aspect of networks.  There
   are at least three common causes for them.  First, any broadcast
   interface is represented by a pseudo-node and has asymmetric link
   costs to and from that pseudo-node.  Second, when routers come up or
   a link with LDP comes up, it is recommended in [RFC5443] and
   [RFC6987] that the link metric be raised to the maximum cost; this
   may not be symmetric and for [RFC6987] is not expected to be.  Third,
   techniques such as IGP metric tuning for traffic-engineering can
   result in asymmetric link costs.  A fast-reroute solution needs to
   handle network topologies with asymmetric link costs.

   When a network needs to use a micro-loop prevention mechanism
   [RFC5715] such as Ordered FIB[RFC6976] or Nearside
   Tunneling[RFC5715], then the whole IGP area needs to have alternates
   available so that the micro-loop prevention mechanism, which requires
   slower network convergence, can take the necessary time without
   adversely impacting traffic.  Without complete coverage, traffic to
   the unprotected destinations will be dropped for significantly longer



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   than with current convergence - where routers individually converge
   as fast as possible.  See Section 12.1 for more discussion of micro-
   loop prevention and MRTs.

1.2.  Partial Deployment and Backwards Compatibility

   MRT-FRR supports partial deployment.  Routers advertise their ability
   to support MRT.  Inside the MRT-capable connected group of routers
   (referred to as an MRT Island), the MRTs are computed.  Alternates to
   destinations outside the MRT Island are computed and depend upon the
   existence of a loop-free neighbor of the MRT Island for that
   destination.  MRT Islands are discussed in detail in Section 7, and
   partial deployment is discussed in more detail in Section 14.5.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Terminology

   network graph:   A graph that reflects the network topology where all
      links connect exactly two nodes and broadcast links have been
      transformed into the standard pseudo-node representation.

   cut-link:   A link whose removal partitions the network.  A cut-link
      by definition must be connected between two cut-vertices.  If
      there are multiple parallel links, then they are referred to as
      cut-links in this document if removing the set of parallel links
      would partition 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
      that requires two nodes to be removed before the network is
      partitioned.

   2-connected cluster:   A maximal set of nodes that are 2-connected.

   block:   Either a 2-connected cluster, a cut-edge, or an isolated
      vertex.

   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
      path from the same node X to the root along the second tree.
      Redundant trees can always be computed in 2-connected graphs.




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   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 the same node X to the root along the second tree share the
      minimum number of nodes and the minimum number of links.  Each
      such shared node is a cut-vertex.  Any shared links are cut-links.
      In graphs that are not 2-connected, it is not possible to compute
      RTs.  However, it is possible to compute MRTs.  MRTs are maximally
      redundant in the sense that they are as redundant as possible
      given the constraints of the network graph.

   DAG:   Directed Acyclic Graph - a graph where all links are directed
      and there are no cycles in it.

   ADAG:   Almost Directed Acyclic Graph - a graph that, if all links
      incoming to the root were removed, would be a DAG.

   GADAG:   Generalized ADAG - a graph that is the combination of the
      ADAGs of all blocks.

   MRT-Red:   MRT-Red is used to describe one of the two MRTs; it is
      used to describe the associated forwarding topology and MPLS
      multi-topology identifier (MT-ID).  Specifically, MRT-Red is the
      decreasing MRT where links in the GADAG are taken in the direction
      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
      used to described the associated forwarding topology and MPLS MT-
      ID.  Specifically, MRT-Blue is the increasing MRT where links in
      the GADAG are taken in the direction from a lower topologically
      ordered node to a higher one.

   Rainbow MRT:   It is useful to have an MPLS MT-ID that refers to the
      multiple MRT forwarding topologies and to the default forwarding
      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
      always be advertised to all peers for the same (MT-ID, Prefix).

   MRT Island:   The set of routers that support a particular MRT
      profile and the links connecting them that support MRT.

   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
      in a common area or level.

   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.





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   named proxy-node:   A proxy-node can represent a destination prefix
      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
      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 IP addresses are advertised in an undefined fashion for that
      proxy-node.

4.  Maximally Redundant Trees (MRT)

   A pair of Maximally Redundant Trees is a pair of directed spanning
   trees that provides maximally disjoint paths towards their common
   root.  Only links or nodes whose failure would partition the network
   (i.e. cut-links and cut-vertices) are shared between the trees.  The
   MRT Lowpoint algorithm is given in
   [I-D.ietf-rtgwg-mrt-frr-algorithm].  This algorithm can be computed
   in O(e + n log n); it is less than three SPFs.  This document
   describes how the MRTs can be used and not how to compute them.

   MRT provides destination-based trees for each destination.  Each
   router stores its normal primary next-hop(s) as well as MRT-Blue
   next-hop(s) and MRT-Red next-hop(s) toward each destination.  The
   alternate will be selected between the MRT-Blue and MRT-Red.

   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
   the destination D on the Blue MRT that is as disjoint as possible
   from the path on the Red MRT.

   For example, in Figure 2, there is a network graph that is
   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
   B->F->D->E->R or B->C->D->E->R.  On the Red MRT, the path is B->A->R.
   These are clearly link and node-disjoint.  These MRTs are redundant
   trees because the paths are disjoint.

   [E]---[D]---|           [E]<--[D]<--|                [E]-->[D]---|
    |     |    |            |     ^    |                       |    |
    |     |    |            V     |    |                       V    V
   [R]   [F]  [C]          [R]   [F]  [C]               [R]   [F]  [C]
    |     |    |                  ^    ^                 ^     |    |
    |     |    |                  |    |                 |     V    |
   [A]---[B]---|           [A]-->[B]---|                [A]<--[B]<--|

         (a)                     (b)                         (c)
   a 2-connected graph     Blue MRT towards R          Red MRT towards R

                      Figure 2: A 2-connected Network



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   By contrast, in Figure 3, the network in (a) is not 2-connected.  If
   F, G or the link F<->G failed, then the network would be partitioned.
   It is clearly impossible to have two link-disjoint or node-disjoint
   paths from G, I or J to R.  The MRTs given in (b) and (c) offer paths
   that are as disjoint as possible.  For instance, the paths from B to
   R are the same as in Figure 2 and the path from G to R on the Blue
   MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.


                      [E]---[D]---|
                       |     |    |     |----[I]
                       |     |    |     |     |
                      [R]---[C]  [F]---[G]    |
                       |     |    |     |     |
                       |     |    |     |----[J]
                      [A]---[B]---|

                                  (a)
                        a non-2-connected graph

       [E]<--[D]<--|                        [E]-->[D]
        |     ^    |          [I]                  |          |----[I]
        V     |    |           |                   V          V     ^
       [R]   [C]  [F]<--[G]    |            [R]<--[C]  [F]<--[G]    |
              ^    ^     ^     V             ^          |           |
              |    |     |----[J]            |          |          [J]
       [A]-->[B]---|                        [A]<--[B]<--|

                   (b)                                    (c)
            Blue MRT towards R                    Red MRT towards R


                    Figure 3: A non-2-connected network

5.  Maximally Redundant Trees (MRT) and Fast-Reroute

   In normal IGP routing, each router has its shortest path tree (SPT)to
   all destinations.  From the perspective of a particular destination,
   D, this looks like a reverse SPT.  To use maximally redundant trees,
   in addition, each destination D has two MRTs associated with it; by
   convention these will be called the MRT-Blue and MRT-Red.  MRT-FRR is
   realized by using multi-topology forwarding.  There is a MRT-Blue
   forwarding topology and a MRT-Red forwarding topology.

   Any IP/LDP fast-reroute technique beyond LFA requires an additional
   dataplane procedure, such as an additional forwarding mechanism.  The
   well-known options are multi-topology forwarding (used by MRT-FRR),
   tunneling (e.g.  [RFC6981] or [RFC7490]), and per-interface



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   forwarding (e.g.  Loop-Free Failure Insensitive Routing in
   [EnyediThesis]).

   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 MRTs to reach the destination D.  For example, in Figure 3, C
   would normally forward traffic to R across the C<->R link.  If that
   C<->R link fails, then C could use the Blue MRT path C->D->E->R.

   As is always the case with fast-reroute technologies, forwarding does
   not change until a local failure is detected.  Packets are forwarded
   along the shortest path.  The appropriate alternate to use is pre-
   computed.  [I-D.ietf-rtgwg-mrt-frr-algorithm] describes exactly how
   to determine whether the MRT-Blue next-hops or the MRT-Red next-hops
   should be the MRT alternate next-hops for a particular primary next-
   hop to a particular destination.

   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
   type of alternate.

   As described in [RFC5286], when a worse failure than is anticipated
   happens, using LFAs that are not downstream neighbors can cause
   looping among alternates.  Section 1.1 of [RFC5286] gives an example
   of link-protecting alternates causing a loop on node failure.  Even
   if a worse failure than anticipated happens, the use of MRT
   alternates will not cause looping.

6.  Unicast Forwarding with MRT Fast-Reroute

   There are three possible types of routers involved in forwarding a
   packet along an MRT path.  At the MRT ingress router, the packet
   leaves the shortest path to the destination and follows an MRT path
   to the destination.  In a FRR application, the MRT ingress router is
   the PLR.  An MRT transit router takes a packet that arrives already
   associated with the particular MRT, and forwards it on that same MRT.
   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
   at the MRT egress router.  The MRT ingress and egress functionality
   may depend on the underlying type of packet being forwarded (LDP or
   IP).  The MRT transit functionality is independent of the type of
   packet being forwarded.  We first consider several MRT transit
   forwarding mechanisms.  Then we look at how these forwarding
   mechanisms can be applied to carrying LDP and IP traffic.







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6.1.  MRT Forwarding Mechanisms

   The following options for MRT forwarding mechanisms are considered.

   1.  MRT LDP Labels

       A.  Topology-scoped FEC encoded using a single label

       B.  Topology and FEC encoded using a two label stack

   2.  MRT IP Tunnels

       A.  MRT IPv4 Tunnels

       B.  MRT IPv6 Tunnels

6.1.1.  MRT LDP labels

   We consider two options for the MRT forwarding mechanisms using MRT
   LDP labels.

6.1.1.1.  Topology-scoped FEC encoded using a single label (Option 1A)

   [RFC7307] provides a mechanism to distribute FEC-Label bindings
   scoped to a given MPLS topology (represented by MPLS MT-ID).  To use
   multi-topology LDP to create MRT forwarding topologies, we associate
   two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies,
   in addition to the default shortest path forwarding topology with MT-
   ID=0.

   With this forwarding mechanism, a single label is distributed for
   each topology-scoped FEC.  For a given FEC in the default topology
   (call it default-FEC-A), two additional topology-scoped FECs would be
   created, corresponding to the Red and Blue MRT forwarding topologies
   (call them red-FEC-A and blue-FEC-A).  A router supporting this MRT
   transit forwarding mechanism advertises a different FEC-label binding
   for each of the three topology-scoped FECs.  When a packet is
   received with a label corresponding to red-FEC-A (for example), an
   MRT transit router will determine the next-hop for the MRT-Red
   forwarding topology for that FEC, swap the incoming label with the
   outgoing label corresponding to red-FEC-A learned from the MRT-Red
   next-hop router, and forward the packet.

   This forwarding mechanism has the useful property that the FEC
   associated with the packet is maintained in the labels at each hop
   along the MRT.  We will take advantage of this property when
   specifying how to carry LDP traffic on MRT paths using multi-topology
   LDP labels.



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   This approach is very simple for hardware to support.  However, it
   reduces the label space for other uses, and it increases the memory
   needed to store the labels and the communication required by LDP to
   distribute FEC-label bindings.

   This forwarding option uses the LDP signaling extensions described in
   [RFC7307].  The MRT-specific LDP extensions required to support this
   option will be described elsewhere.

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
   the topology and the FEC of the packet.  The top label (topology-id
   label) identifies the MRT forwarding topology, while the second label
   (FEC label) identifies the FEC.  The top label would be a new FEC
   type with two values corresponding to MRT Red and Blue topologies.

   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
   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-
   label bindings learned through normal LDP mechanisms.  The router
   then pushes the topology-id label for the next-hop.

   As with Option 1A, this forwarding mechanism also has the useful
   property that the FEC associated with the packet is maintained in the
   labels at each hop along the MRT.

   This forwarding mechanism has minimal usage of additional labels,
   memory and LDP communication.  It does increase the size of packets
   and the complexity of the required label operations and look-ups.

   This forwarding option is consistent with context-specific label
   spaces, as described in [RFC5331].  However, the precise LDP behavior
   required to support this option for MRT has not been specified.

6.1.1.3.  Compatibility of Option 1A and 1B

   In principle, MRT transit forwarding mechanisms 1A and 1B can coexist
   in the same network, with a packet being forwarding along a single
   MRT path using the single label of option 1A for some hops and the
   two label stack of option 1B for other hops.

6.1.1.4.  Mandatory support for MRT LDP Label option 1A

   If a router supports a profile that includes the MRT LDP Label option
   for MRT transit forwarding mechanism, then it MUST support option 1A,
   which encodes topology-scoped FECs using a single label.



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6.1.2.  MRT IP tunnels (Options 2A and 2B)

   IP tunneling can also be used as an MRT transit forwarding mechanism.
   Each router supporting this MRT transit forwarding mechanism
   announces two additional loopback addresses and their associated MRT
   color.  Those addresses are used as destination addresses for MRT-
   blue and MRT-red IP tunnels respectively.  The special loopback
   addresses allow the transit nodes to identify the traffic as being
   forwarded along either the MRT-blue or MRT-red topology to reach the
   tunnel destination.  For example, an MRT ingress router can cause a
   packet to be tunneled along the MRT-red path to router X by
   encapsulating the packet using the MRT-red loopback address
   advertised by router X.  Upon receiving the packet, router X would
   remove the encapsulation header and forward the packet based on the
   original destination address.

   Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the
   tunneling mechanism.

   Note that the two forwarding mechanisms using LDP Label options do
   not require additional loopbacks per router, as is required by the IP
   tunneling mechanism.  This is because LDP labels are used on a hop-
   by-hop basis to identify MRT-blue and MRT-red forwarding topologies.

6.2.  Forwarding LDP Unicast Traffic over MRT Paths

   In the previous section, we examined several options for providing
   MRT transit forwarding functionality, which is independent of the
   type of traffic being carried.  We now look at the MRT ingress
   functionality, which will depend on the type of traffic being carried
   (IP or LDP).  We start by considering LDP traffic.

   We also simplify the initial discussion by assuming that the network
   consists of a single IGP area, and that all routers in the network
   participate in MRT.  Other deployment scenarios that require MRT
   egress functionality are considered later in this document.

   In principle, it is possible to carry LDP traffic in MRT IP tunnels.
   However, for LDP traffic, it is desirable to avoid tunneling.
   Tunneling LDP traffic to a remote node requires knowledge of remote
   FEC-label bindings so that the LDP traffic can continue to be
   forwarded properly when it leaves the tunnel.  This requires targeted
   LDP sessions which can add management complexity.  As described
   below, the two MRT forwarding mechanisms that use LDP labels do not
   require targeted LDP sessions.






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6.2.1.  Forwarding LDP traffic using MRT LDP Labels (Option 1A)

   The MRT LDP Label option 1A forwarding mechanism uses topology-scoped
   FECs encoded using a single label as described in section
   Section 6.1.1.1.  When a PLR receives an LDP packet that needs to be
   forwarded on the Red MRT (for example), it does a label swap
   operation, replacing the usual LDP label for the FEC with the Red MRT
   label for that FEC received from the next-hop router in the Red MRT
   computed by the PLR.  When the next-hop router in the Red MRT
   receives the packet with the Red MRT label for the FEC, the MRT
   transit forwarding functionality continues as described in
   Section 6.1.1.1.  In this way the original FEC associated with the
   packet is maintained at each hop along the MRT.

6.2.2.  Forwarding LDP traffic using MRT LDP Labels (Option 1B)

   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.
   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
   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
   the Red MRT.  In addition, the PLR pushes the topology-identification
   label associated with the Red MRT, and forward the packet to the
   appropriate next-hop on the Red MRT.  When the next-hop router in the
   Red MRT receives the packet with the Red MRT label for the FEC, the
   MRT transit forwarding functionality continues as described in
   Section 6.1.1.2.  As with option 1A, the original FEC associated with
   the packet is maintained at each hop along the MRT.

6.2.3.  Other considerations for forwarding LDP traffic using MRT LDP
        Labels

   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
   destination FEC is used.  The alternates selected in
   [I-D.ietf-rtgwg-mrt-frr-algorithm] use the MRT path to the
   destination FEC, so targeted LDP sessions are not needed.  If instead
   one found it desirable to have the PLR use an MRT to reach the
   primary next-next-hop for the FEC, and then continue forwarding the
   LDP packet along the shortest path tree from the primary next-next-
   hop, this would require tunneling to the primary next-next-hop and a
   targeted LDP session for the PLR to learn the FEC-label binding for
   primary next-next-hop to correctly forward the packet.

   For greatest hardware compatibility, routers implementing MRT fast-
   reroute of LDP traffic MUST support Option 1A of encoding the MT-ID
   in the labels (See Section 9).



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6.3.  Forwarding IP Unicast Traffic over MRT Paths

   For IPv4 traffic, there is no currently practical alternative except
   tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
   forwarding topology.  For IPv6 traffic, in principle one could define
   bits in the IPv6 options header to indicate the MRT-Blue or MRT-Red
   forwarding topology.  However, in this document, we have chosen not
   to define a solution that would work for IPv6 traffic but not for
   IPv4 traffic.

   The choice of tunnel egress is flexible since any router closer to
   the destination than the next-hop can work.  This architecture
   assumes that the original destination in the area is selected (see
   Section 11 for handling of multi-homed prefixes); another possible
   choice is the next-next-hop towards the destination.  As discussed in
   the previous section, for LDP traffic, using the MRT to the original
   destination simplifies MRT-FRR by avoiding the need for targeted LDP
   sessions to the next-next-hop.  For IP, that consideration doesn't
   apply.

   Some situations require tunneling IP traffic along an MRT to a tunnel
   endpoint that is not the destination of the IP traffic.  These
   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
   PLR should be tunneled on the MRT to the ABR/LBR on the shortest path
   to the destination.  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 named proxy-node on that particular color MRT.

6.3.1.  Tunneling IP traffic using MRT LDP Labels

   An IP packet can be tunneled along an MRT path by pushing the
   appropriate MRT LDP label(s).  Tunneling using LDP labels, as opposed
   to IP headers, has the the advantage that more installed routers can
   do line-rate encapsulation and decapsulation using LDP than using IP.
   Also, no additional IP addresses would need to be allocated or
   signaled.

6.3.1.1.  Tunneling IP traffic using MRT LDP Labels (Option 1A)

   The MRT LDP Label option 1A forwarding mechanism uses topology-scoped
   FECs encoded using a single label as described in section
   Section 6.1.1.1.  When a PLR receives an IP packet that needs to be
   forwarded on the Red MRT to a particular tunnel endpoint, it does a
   label push operation.  The label pushed is the Red MRT label for a
   FEC originated by the tunnel endpoint, learned from the next-hop on
   the Red MRT.




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6.3.1.2.  Tunneling IP traffic using MRT LDP Labels (Option 1B)

   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.
   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
   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
   originated by the tunnel endpoint.  The second (outer) label is the
   topology-identification label associated with the Red MRT.

   For completeness, we note here a potential variation that uses a
   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
   an arbitrary tunnel endpoint), then we could just push a topology-
   identification label directly onto the packet.  An MRT transit router
   would need to pop the topology-id label, do an IP route lookup in the
   context of that topology-id , and push the topology-id label.

6.3.2.  Tunneling IP traffic using MRT IP Tunnels

   In order to tunnel over the MRT to a particular tunnel endpoint, the
   PLR encapsulates the original IP packet with an additional IP header
   using the MRT-Blue or MRT-Red loopack address of the tunnel endpoint.

6.3.3.  Required support

   For greatest hardware compatibility and ease in removing the MRT-
   topology marking at area/level boundaries, routers that support MPLS
   and implement IP MRT fast-reroute MUST support tunneling of IP
   traffic using MRT LDP Labels Option 1A (topology-scoped FEC encoded
   using a single label).

7.  MRT Island Formation

   The purpose of communicating support for MRT is to indicate that the
   MRT-Blue and MRT-Red forwarding topologies are created for transit
   traffic.  The MRT architecture allows for different, potentially
   incompatible options.  In order to create consistent MRT forwarding
   topologies, the routers participating in a particular MRT Island need
   to use the same set of options.  These options are grouped into MRT
   profiles.  In addition, the routers in an MRT Island all need to use
   the same set of nodes and links within the Island when computing the
   MRT forwarding topologies.  This section describes the information
   used by a router to determine the nodes and links to include in a
   particular MRT Island.  Some information already exists in the IGPs
   and can be used by MRT in Island formation, subject to the
   interpretation defined here.



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   Other information needs to be communicated between routers for which
   there do not currently exist protocol extensions.  This new
   information needs to be shared among all routers in an IGP area, so
   defining extensions to existing IGPs to carry this information makes
   sense.  These new protocol extensions will be defined elsewhere.

   Deployment scenarios using multi-topology OSPF or IS-IS, or running
   both ISIS and OSPF on the same routers is out of scope for this
   specification.  As with LFA, it is expected that OSPF Virtual Links
   will not be supported.

7.1.  IGP Area or Level

   All links in an MRT Island MUST be bidirectional and belong to the
   same IGP area or level.  For ISIS, a link belonging to both level 1
   and 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 levels in which it participates.  Inter-area forwarding behavior
   is discussed in Section 10.

7.2.  Support for a specific MRT profile

   All routers in an MRT Island MUST support the same MRT profile.  A
   router 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 document.  The protocol extensions for advertising the MRT
   Profile ID value will be defined elsewhere.  A given router can
   support multiple MRT profiles and participate in multiple MRT
   Islands.  The options that make up an MRT profile, as well as the
   default MRT profile, are defined in Section 8.

   Note that a router may advertise support for multiple different MRT
   profiles.  The process of MRT Island formation takes place
   independently for each MRT profile advertised by a given router.  For
   example, consider a network with 40 connected routers in the same
   area advertising support for MRT Profile A and MRT Profile B.  Two
   distinct MRT Islands will be formed corresponding to Profile A and
   Profile B, with each island containing all 40 routers.  A complete
   set of maximally redundant trees will be computed for each island
   following the rules 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 routers, there will be a third MRT Island
   formed corresponding to those 30 routers, and a third set of
   maximally redundant trees will be computed.  In this example, 40
   routers would compute and install two sets of MRT transit forwarding
   entries corresponding to Profiles A and B, while 30 routers would
   compute and install three sets of MRT transit forwarding entries
   corresponding to Profiles A, B, and C.



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7.3.  Excluding additional routers and interfaces from the MRT Island

   MRT takes into account existing IGP mechanisms for discouraging
   traffic from using particular links and routers, and it introduces an
   MRT-specific exclusion mechanism for links.

7.3.1.  Existing IGP exclusion mechanisms

   Mechanisms for discouraging traffic from using particular links
   already exist in ISIS and OSPF.  In ISIS, an interface configured
   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)
   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
   last resort.  These metrics can be configured manually to enforce
   administrative policy, or they can be set in an automated manner as
   with LDP IGP synchronization [RFC5443].

   Mechanisms also already exist in ISIS and OSPF to discourage or
   prevent transit traffic from using a particular router.  In ISIS, the
   overload bit is prevents transit traffic from using a router.

   For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing
   interface metrics to 0xFFFF to discourage transit traffic from using
   a router.( [RFC6987] defines the metric value 0xFFFF as
   MaxLinkMetric, a fixed architectural value for OSPF.)  For OSPFv3,
   [RFC5340] specifies that a router be excluded from the intra-area
   shortest path tree computation if the V6-bit or R-bit of the LSA
   options is not set in the Router LSA.

   The following rules for MRT Island formation ensure that MRT FRR
   protection traffic does not use a link or router that is discouraged
   or prevented from carrying traffic by existing IGP mechanisms.

   1.  A bidirectional link MUST be excluded from an MRT Island if
       either the forward or reverse cost on the link is 0xFFFFFE (for
       ISIS) or 0xFFFF for OSPF.

   2.  A router MUST be excluded from an MRT Island if it is advertised
       with the overload bit set (for ISIS), or it is advertised with
       metric values of 0xFFFF on all of its outgoing interfaces (for
       OSPFv2 and OSPFv3).

   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
       Router LSA.





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7.3.2.  MRT-specific exclusion mechanism

   This architecture also defines a means of excluding an otherwise
   usable link from MRT Islands.  The protocol extensions for
   advertising that a link is MRT-Ineligible will be defined elsewhere.
   A link with either interface advertised as MRT-Ineligible MUST be
   excluded from an MRT Island.  Note that an interface advertised as
   MRT-Ineligible by a router is ineligible with respect to all profiles
   advertised by that router.

7.4.  Connectivity

   All of the routers in an MRT Island MUST be connected by
   bidirectional links with other routers in the MRT Island.
   Disconnected MRT Islands will operate independently of one another.

7.5.  Algorithm for MRT Island Identification

   An algorithm that allows a computing router to identify the routers
   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].

8.  MRT Profile

   An MRT Profile is a set of values and options related to MRT
   behavior.  The complete set of options is designated by the
   corresponding 8-bit Profile ID value.

   This document specifies the values and options that correspond to the
   Default MRT Profile (Profile ID = 0).  Future documents may define
   other MRT Profiles by specifying the MRT Profile Options below.

8.1.  MRT Profile Options

   Below is a description of the values and options that define an MRT
   Profile.

   MRT Algorithm:   This identifies the particular algorithm for
      computing maximally redundant trees used by the router for this
      profile.

   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
      Multi-Topology Identifiers Registry.

   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
      Multi-Topology Identifiers Registry.



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   GADAG Root Selection Policy:   This specifies the manner in which the
      GADAG root is selected.  All routers in the MRT island need to use
      the same GADAG root in the calculations used construct the MRTs.
      A valid GADAG Root Selection Policy MUST be such that each router
      in the MRT island chooses the same GADAG root based on information
      available to all routers in the MRT island.  GADAG Root Selection
      Priority values, advertised as router-specific MRT parameters, MAY
      be used in a GADAG Root Selection Policy.

   MRT Forwarding Mechanism:   This specifies which forwarding mechanism
      the router uses to carry transit traffic along MRT paths.  A
      router which supports a specific MRT forwarding mechanism must
      program appropriate next-hops into the forwarding plane.  The
      current options are MRT LDP Labels, IPv4 Tunneling, IPv6
      Tunneling, and None.  If the MRT LDP Labels option is supported,
      then option 1A and the appropriate signaling extensions MUST be
      supported.  If IPv4 is supported, then both MRT-Red and MRT-Blue
      IPv4 Loopback Addresses SHOULD be specified.  If IPv6 is
      supported, both MRT-Red and MRT-Blue IPv6 Loopback Addresses
      SHOULD be specified.

   Recalculation:   Recalculation specifies the process and timing by
      which new MRTs are computed after the topology has been modified.

   Area/Level Border Behavior:   This specifies how traffic traveling on
      the MRT-Blue or MRT-Red in one area should be treated when it
      passes into another area.

   Other Profile-Specific Behavior:   Depending upon the use-case for
      the profile, there may be additional profile-specific behavior.

   When a new MRT Profile is defined, new and unique values should be
   allocated from the MPLS Multi-Topology Identifiers Registry,
   corresponding to the MRT-Red and MRT-Blue MT-ID values for the new
   MRT Profile .

   If a router advertises support for multiple MRT profiles, then it
   MUST create the transit forwarding topologies for each of those,
   unless the profile specifies the None option for MRT Forwarding
   Mechanism.

   The ability of MRT-FRR to support transit forwarding entries for
   multiple profiles can be used to facilitate a smooth transition from
   an existing deployed MRT Profile to a new MRT Profile.  The new
   profile can be activated in parallel with the existing profile,
   installing the transit forwarding entries for the new profile without
   affecting the transit forwarding entries for the existing profile.
   Once the new transit forwarding state has been verified, the router



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   can be configured to use the alternates computed by the new profile
   in the event of a failure.

8.2.  Router-specific MRT paramaters

   For some profiles, additional router-specific MRT parameters may need
   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
   router-specific MRT parameters may differ between routers in the same
   MRT island.  Several such parameters are described below.

   GADAG Root Selection Priority:   A GADAG Root Selection Policy MAY
      rely on the GADAG Root Selection Priority values advertised by
      each router in the MRT island.  A GADAG Root Selection Policy may
      use the GADAG Root Selection Priority to allow network operators
      to configure a parameter to ensure that the GADAG root is selected
      from a particular subset of routers.  An example of this use of
      the GADAG Root Selection Priority value by the GADAG Root
      Selection Policy is given in the Default MRT profile below.

   MRT-Red Loopback Address:   This provides the router's loopback
      address to reach the router via the MRT-Red forwarding topology.
      It can be specified for either IPv4 or IPv6.  Note that this
      parameter is not needed to support the Default MRT profile.

   MRT-Blue Loopback Address:   This provides the router's loopback
      address to reach the router via the MRT-Blue forwarding topology.
      It can be specified for either IPv4 and IPv6.  Note that this
      parameter is not needed to support the Default MRT profile.

   Protocol extensions for advertising a router's GADAG Root Selection
   Priority value will be defined in other documents.  Protocol
   extensions for the advertising a router's MRT-Red and MRT-Blue
   Loopback Addresses will be defined elsewhere.

8.3.  Default MRT profile

   The following set of options defines the default MRT Profile.  The
   default MRT profile is indicated by the MRT Profile ID value of 0.

   MRT Algorithm:   MRT Lowpoint algorithm defined in
      [I-D.ietf-rtgwg-mrt-frr-algorithm].

   MRT-Red MPLS MT-ID:   This value will be allocated from the MPLS
      Multi-Topology Identifiers Registry.  The IANA request for this
      allocation will be in another document.





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   MRT-Blue MPLS MT-ID:   This value will be allocated from the MPLS
      Multi-Topology Identifiers Registry.  The IANA request for this
      allocation will be in another document.

   GADAG Root Selection Policy:   Among the routers in the MRT Island
      and with the most preferred GADAG Root Selection Priority
      advertised (corresponding to the lowest numerical value of GADAG
      Root Selection Priority), an implementation MUST pick the router
      with the highest Router ID to be the GADAG root.

   Forwarding Mechanisms:   MRT LDP Labels

   Recalculation:   Recalculation of MRTs SHOULD occur as described in
      Section 12.2.  This allows the MRT forwarding topologies to
      support IP/LDP fast-reroute traffic.

   Area/Level Border Behavior:   As described in Section 10, ABRs/LBRs
      SHOULD ensure that traffic leaving the area also exits the MRT-Red
      or MRT-Blue forwarding topology.

9.  LDP signaling extensions and considerations

   The protocol extensions for LDP will be defined in another document.
   A router must indicate that it has the ability to support MRT; having
   this explicit allows the use of MRT-specific processing, such as
   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
   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
   MUST still be sent (even though it could be inferred from the Rainbow
   FEC-label bindings) to ensure continuous operation of normal LDP
   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
   avoids the problem of needing to signal different MPLS labels to
   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
   MRT profile specific.

   The value of the Rainbow MRT MPLS MT-ID will be allocated from the
   MPLS Multi-Topology Identifiers Registry.  The IANA request for this
   allocation will be in another document.

10.  Inter-area Forwarding Behavior

   An ABR/LBR has two forwarding roles.  First, it forwards traffic
   within areas.  Second, it forwards traffic from one area into
   another.  These same two roles apply for MRT transit traffic.



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   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
   traffic leaving the area to also exit MRT-Red or MRT-Blue and return
   to shortest path forwarding.

   For unicast MRT-FRR, the need to stay on an MRT forwarding topology
   terminates at the ABR/LBR whose best route is via a different area/
   level.  It is highly desirable to go back to the default forwarding
   topology when leaving an area/level.  There are three basic reasons
   for this.  First, the default topology uses shortest paths; the
   packet will thus take the shortest possible route to the destination.
   Second, this allows failures that might appear in multiple areas
   (e.g.  ABR/LBR failures) to be separately identified and repaired
   around.  Third, the packet can be fast-rerouted again, if necessary,
   due to a failure in a different area.

   An ABR/LBR that receives a packet on MRT-Red or MRT-Blue towards
   destination Z should continue to forward the packet along MRT-Red or
   MRT-Blue only if the best route to Z is in the same area as the
   interface that the packet was received on.  Otherwise, the packet
   should be removed from MRT-Red or MRT-Blue and forwarded on the
   shortest-path default forwarding topology.

   To avoid per-interface forwarding state for MRT-Red and MRT-Blue, the
   ABR/LBR needs to arrange that packets destined to a different area
   arrive at the ABR/LBR already not marked as MRT-Red or MRT-Blue.

10.1.  ABR Forwarding Behavior with MRT LDP Label Option 1A

   For LDP forwarding where a single label specifies (MT-ID, FEC), the
   ABR/LBR is responsible for advertising the proper label to each
   neighbor.  Assume that an ABR/LBR has allocated three labels for a
   particular destination; those labels are L_primary, L_blue, and
   L_red.  To those routers in the same area as the best route to the
   destination, the ABR/LBR advertises the following FEC-label bindings:
   L_primary for the default topology, L_blue for the MRT-Blue MT-ID and
   L_red for the MRT-Red MT-ID, as expected.  However, to routers in
   other areas, the ABR/LBR advertises the following FEC-label bindings:
   L_primary for the default topology, and L_primary for the Rainbow MRT
   MT-ID.  Associating L_primary with the Rainbow MRT MT-ID causes the
   receiving routers to use L_primary for the MRT-Blue MT-ID and for the
   MRT-Red MT-ID.

   The ABR/LBR 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_red.  Because the ABR/LBR advertised (Rainbow MRT MT-ID,
   FEC) with L_primary to neighbors not in the best area, packets from
   those neighbors will arrive at the ABR/LBR with a label L_primary and



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   will be forwarded into the best area along the default topology.  By
   controlling what labels are advertised, the ABR/LBR can thus enforce
   that packets exiting the area do so on the shortest-path default
   topology.

10.1.1.  Motivation for Creating the Rainbow-FEC

   The desired forwarding behavior could be achieved in the above
   example without using the Rainbow-FEC.  This could be done by having
   the ABR/LBR advertise the following FEC-label bindings to neighbors
   not 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 would require machinery to spoof the labels used in FEC-label
   binding advertisements on a per-neighbor basis.  Such label-spoofing
   machinery does not currently exist in most LDP implmentations and
   doesn't have other obvious uses.

   Many existing LDP implmentations do however have the ability to
   filter FEC-label binding advertisements on a per-neighbor basis.  The
   Rainbow-FEC allows us to re-use the existing per-neighbor FEC
   filtering machinery to achieve the desired result.  By introducing
   the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
   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.

   The use of the Rainbow-FEC by the ABR for non-best-area
   advertisements is RECOMMENDED.  An ABR MAY advertise the label for
   the default topology in separate MRT-Blue and MRT-Red advertisements.
   However, a router that supports the LDP Label MRT Forwarding
   Mechanism MUST be able to receive and correctly interpret the
   Rainbow-FEC.

10.2.  ABR Forwarding Behavior with IP Tunneling (option 2)

   If IP tunneling is used, then the ABR/LBR behavior is dependent upon
   the outermost IP address.  If the outermost IP address is an MRT
   loopback address of the ABR/LBR, then the packet is decapsulated and
   forwarded 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 of the ABR/LBR, then the packet is simply forwarded
   along the associated forwarding topology.  A PLR sending traffic to a
   destination outside its local area/level will pick the MRT and use
   the associated MRT loopback address of the selected ABR/LBR
   advertising the lowest cost to the external destination.

   Thus, for these two MRT Forwarding Mechanisms (MRT LDP Label option
   1A and IP tunneling option 2), there is no need for additional
   computation or per-area forwarding state.



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10.3.  ABR Forwarding Behavior with LDP Label option 1B

   The other MRT forwarding mechanism described in Section 6 uses two
   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/
   LBR would need to determine whether the ABR/LBR would forward the
   packet out of the area/level.  If so, then that router should pop off
   the topology-identification label before forwarding the packet to the
   ABR/LBR.

   For example, in Figure 4, if node H fails, node E has to put traffic
   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
   Topology-Identification Label and just forward the packet to ABR1
   along the MRT-Red next-hop.  ABR1 will use the shortest path in Area
   10.

   In all cases for ISIS and most cases for OSPF, the penultimate 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 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 the largest OPSF
   area ID) and the Area ID isn't announced as part of the ASBR link-
   state advertisement (LSA).  In this one case, suboptimal forwarding
   along the MRT in the other area would happen.  If that becomes a
   realistic deployment scenario, protocol extensions could be developed
   to address this issue.
























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       +----[C]----     --[D]--[E]                --[D]--[E]
       |           \   /         \               /         \
   p--[A] Area 10 [ABR1]  Area 0 [H]--p   +-[ABR1]  Area 0 [H]-+
       |           /   \         /        |      \         /   |
       +----[B]----     --[F]--[G]        |       --[F]--[G]   |
                                          |                    |
                                          | other              |
                                          +----------[p]-------+
                                            area

         (a) Example topology        (b) Proxy node view in Area 0 nodes


                   +----[C]<---       [D]->[E]
                   V           \             \
                +-[A] Area 10 [ABR1]  Area 0 [H]-+
                |  ^           /             /   |
                |  +----[B]<---       [F]->[G]   V
                |                                |
                +------------->[p]<--------------+

                  (c) rSPT towards destination p



             ->[D]->[E]                         -<[D]<-[E]
            /          \                       /         \
       [ABR1]  Area 0 [H]-+             +-[ABR1]         [H]
                      /   |             |      \
               [F]->[G]   V             V       -<[F]<-[G]
                          |             |
                          |             |
                [p]<------+             +--------->[p]

     (d) Blue MRT in Area 0           (e) Red MRT in Area 0


                Figure 4: ABR Forwarding Behavior and MRTs

11.  Prefixes Multiply Attached to the MRT Island

   How a computing router S determines its local MRT Island for each
   supported MRT profile is already discussed in Section 7.

   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
   usually connect at a domain or protocol boundary.  The second type
   represent routers that do not support the profile for the MRT Island.



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   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
   not selected.

   FRR using LFA has the useful property that it is able to protect
   multi-homed prefixes against ABR failure.  For instance, if a prefix
   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
   with MRT FRR as well.

   If ASBR protection is desired, this has additional complexities if
   the ASBRs are in different areas.  Similarly, protecting labeled BGP
   traffic in the event of an ASBR failure has additional complexities
   due to the per-ASBR label spaces involved.

   As discussed in [RFC5286], a multi-homed prefix could be:

   o  An out-of-area prefix announced by more than one ABR,

   o  An AS-External route announced by 2 or more ASBRs,

   o  A prefix with iBGP multipath to different ASBRs,

   o  etc.

   See Appendix A for a discussion of a general issue with multi-homed
   prefixes connected in two different areas.

   There are also two different approaches to protection.  The first is
   tunnel endpoint selection where the PLR picks a router to tunnel to
   where that router is loop-free with respect to the failure-point.
   Conceptually, the set of candidate routers to provide LFAs expands to
   all routers that can be reached via an MRT alternate, attached to the
   prefix.

   The second is to use a proxy-node, that can be named via MPLS label
   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
   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
   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
   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
   proxy-node.  Traffic to a named proxy-node may take a different path
   than traffic to the attaching router; traffic is also explicitly
   forwarded from the attaching router along a predetermined interface
   towards the relevant prefixes.



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   For IP traffic, multi-homed prefixes can use tunnel endpoint
   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
   the MRT Island, then the named proxy-node approach can be used.

   For LDP traffic, there is always a FEC advertised into the MRT
   Island.  The named proxy-node approach should be used, unless the
   computing router S knows the label for the FEC at the selected tunnel
   endpoint.

   If a FEC is advertised from outside the MRT Island into the MRT
   Island and the forwarding mechanism specified in the profile includes
   LDP, then the routers learning that FEC MUST also advertise labels
   for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT
   Island.  Any router receiving a FEC corresponding to a router outside
   the MRT Island or to a multi-homed prefix MUST compute and install
   the transit MRT-Blue and MRT-Red next-hops for that FEC.  The FEC-
   label bindings for the topology-scoped FECs ((MT-ID 0, FEC), (MRT-
   Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
   neighbors inside the MRT Island.

11.1.  Protecting Multi-Homed Prefixes using Tunnel Endpoint Selection

   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
   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
   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
   to p.

   The candidates for tunnel endpoint selection are those to which the
   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
   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
   always prefer to send traffic to p via a different area/level, then
   this is definitional.  Otherwise, distance-based computations are
   necessary and an SPF from B's perspective may be necessary.  The
   following equations give the checks needed; the rationale is similar
   to that given in [RFC5286].

   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)





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   The latter is equivalent to the following, which avoids the need to
   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,
   F)

   Finally, the rules for Endpoint selection are given below.  The basic
   idea is to repair to the prefix-advertising router selected for the
   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
   cut-link).

   1.  Does S have a node-protecting alternate to A?  If so, select
       that.  Tunnel the packet to A along that alternate.  For example,
       if LDP is the forwarding mechanism, then push the label (MRT-Red,
       A) or (MRT-Blue, A) onto the packet.

   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.
       Determine the MRT alternate to reach B while avoiding F.  Tunnel
       the packet to B along that alternate.  For example, with LDP,
       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
       so, select that.

   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
       the endpoint and the MRT alternate for reaching B from S that
       avoid the link (S,F).

   The tunnel endpoint selected will receive a packet destined to itself
   and, being the egress, will pop that MPLS label (or have signaled
   Implicit Null) and forward based on what is underneath.  This
   suffices for IP traffic since the tunnel endpoint can use the IP
   header of the original packet to continue forwarding the packet.
   However, tunnelling of LDP traffic requires targeted LDP sessions for
   learning the FEC-label binding at the tunnel endpoint.

11.2.  Protecting Multi-Homed Prefixes using Named Proxy-Nodes

   Instead, the named proxy-node method works with LDP traffic without
   the need for targeted LDP sessions.  It also has a clear advantage
   over tunnel endpoint selection, in that it is possible to explicitly
   forward from the MRT Island along an interface to a loop-free island
   neighbor when that interface may not be a primary next-hop.





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   A named proxy-node represents one or more destinations and, for LDP
   forwarding, has a FEC associated with it that is signalled into the
   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
   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
   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
   uniquely represented by the two routers in the MRT Island to which it
   is connected.  The extensions to signal such IP addresses will be
   defined elsewhere.  The details of what label-bindings must be
   originated will be described in another document.

   Computing the MRT next-hops to a named proxy-node and the MRT
   alternate for the computing router S to avoid a particular failure
   node F is straightforward.  The details of the simple constant-time
   functions, Select_Proxy_Node_NHs() and
   Select_Alternates_Proxy_Node(), are given in
   [I-D.ietf-rtgwg-mrt-frr-algorithm].  A key point is that computing
   these MRT next-hops and alternates can be done as new named proxy-
   nodes are added or removed without requiring a new MRT computation or
   impacting other existing MRT paths.  This maps very well to, for
   example, how OSPFv2 (see [RFC2328] Section 16.5) does incremental
   updates for new summary-LSAs.

   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
   consistently.  No more than 2 routers in the MRT Island can be
   selected; one should only be selected if there are no others that
   meet the necessary criteria.  The named proxy-node is logically part
   of the area/level.

   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
   the MRT Island that are advertising the prefix; the named-proxy-cost
   assigned to each prefix-advertising router is the announced cost to
   the prefix.  The second set are those routers in the MRT Island that
   are connected to routers not in the MRT Island but in the same area/
   level; such routers will be defined as Island Border Routers (IBRs).
   The routers connected to the IBRs that are not in the MRT Island and
   are in the same area/level as the MRT island are Island
   Neighbors(INs).

   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
   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
   a candidate router only if it possesses at least one IN whose



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   shortest path to the prefix does not enter the MRT Island.  A method
   for identifying loop-free Island Neighbors(LFINs) is given in
   [I-D.ietf-rtgwg-mrt-frr-algorithm].  The named-proxy-cost assigned to
   each (IBR, IN) pair is cost(IBR, IN) + D_opt(IN, prefix).

   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
   are selected.  Ties are broken based upon the lowest Router ID.  For
   ease of discussion, the two selected routers will be referred to as
   proxy-node attachment routers.

   A proxy-node attachment router has a special forwarding role.  When a
   packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
   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
   (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
   proxy-node attachment router is not an IBR, then the packet MUST be
   removed from the MRT forwarding topology and sent along the
   interface(s) that caused the router to advertise the prefix; this
   interface might be out of the area/level/AS.

11.3.  MRT Alternates for Destinations Outside the MRT Island

   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
   MRT FRR, where it provides alternates when appropriate LFAs aren't
   available, there are also deployment scenarios where it may make
   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
   is connected via two routers to the rest of the area.

   Destinations inside the local island can obviously use MRT
   alternates.  Destinations outside the local island can be treated
   like a multi-homed prefix and either Endpoint Selection or Named
   Proxy-Nodes can be used.  Named Proxy-Nodes MUST be supported when
   LDP forwarding is supported and a label-binding for the destination
   is sent to an IBR.

   Naturally, there are more complicated options to improve coverage,
   such as connecting multiple MRT islands across tunnels, but the need
   for the additional complexity has not been justified.

12.  Network Convergence and Preparing for the Next Failure

   After a failure, MRT detours ensure that packets reach their intended
   destination while the IGP has not reconverged onto the new topology.
   As link-state updates reach the routers, the IGP process calculates



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   the new shortest paths.  Two things need attention: micro-loop
   prevention and MRT re-calculation.

12.1.  Micro-loop prevention and MRTs

   A micro-loop is a transient packet forwarding loop among two or more
   routers that can occur during convergence of IGP forwarding state.
   [RFC5715] discusses several techniques for preventing micro-loops.
   This section discusses how MRT-FRR relates to two of the micro-loop
   prevention techniques discussed in [RFC5715], Nearside Tunneling and
   Farside Tunneling.

   In Nearside Tunneling, a router (PLR) adjacent to a failure perform
   local repair and inform remote routers of the failure.  The remote
   routers initially tunnel affected traffic to the nearest PLR, using
   tunnels which are unaffected by the failure.  Once the forwarding
   state for normal shortest path routing has converged, the remote
   routers return the traffic to shortest path forwarding.  MRT-FRR is
   relevant for Nearside Tunneling for the following reason.  The
   process of tunneling traffic to the PLRs and waiting a sufficient
   amount of time for IGP forwarding state convergence with Nearside
   Tunneling means that traffic will generally be relying on the local
   repair at the PLR for longer than it would in the absence of Nearside
   Tunneling.  Since MRT-FRR provides 100% coverage for single link and
   node failure, it may be an attractive option to provide the local
   repair paths when Nearside Tunneling is deployed.

   MRT-FRR is also relevant for the Farside Tunneling micro-loop
   prevention technique.  In Farside Tunneling, remote routers tunnel
   traffic affected by a failure to a node downstream of the failure
   with respect to traffic destination.  This node can be viewed as
   being on the farside of the failure with respect to the node
   initiating the tunnel.  Note that the discussion of Farside Tunneling
   in [RFC5715] focuses on the case where the farside node is
   immediately adjacent to a failed link or node.  However, the farside
   node may be any node downstream of the failure with respect to
   traffic destination, including the destination itself.  The tunneling
   mechanism used to reach the farside node must be unaffected by the
   failure.  The alternative forwarding paths created by MRT-FRR have
   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
   of failure, either the MRT-Red or MRT-Blue path from the remote
   upstream router to the destination is guaranteed to avoid a link
   failure or inferred node failure.  The MRT forwarding paths are also
   guaranteed to not be subject to micro-loops because they are locked
   to the topology before the failure.





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   We note that the computations in [I-D.ietf-rtgwg-mrt-frr-algorithm]
   address the case of a PLR adjacent to a failure determining which
   choice of MRT-Red or MRT-Blue will avoid a failed link or node.  More
   computation may be required for an arbitrary remote upstream router
   to determine whether to choose MRT-Red or MRT-Blue for a given
   destination and failure.

12.2.  MRT Recalculation for the Default MRT Profile

   This section describes how the MRT recalculation SHOULD be performed
   for the Default MRT Profile.  This is intended to support FRR
   applications.  Other approaches are possible, but they are not
   specified in this document.

   When a failure event happens, traffic is put by the PLRs onto the MRT
   topologies.  After that, each router recomputes its shortest path
   tree (SPT) and moves traffic over to that.  Only after all the PLRs
   have switched to using their SPTs and traffic has drained from the
   MRT topologies should each router install the recomputed MRTs into
   the FIBs.

   At each router, therefore, the sequence is as follows:

   1.  Receive failure notification

   2.  Recompute SPT.

   3.  Install the new SPT in the FIB.

   4.  If the network was stable before the failure occured, wait a
       configured (or advertised) period for all routers to be using
       their SPTs and traffic to drain from the MRTs.

   5.  Recompute MRTs.

   6.  Install new MRTs in the FIB.

   While the recomputed MRTs are not installed in the FIB, protection
   coverage is lowered.  Therefore, it is important to recalculate the
   MRTs and install them quickly.

   New protocol extensions for advertising the time needed to recompute
   shortest path routes and install them in the FIB will be defined
   elsewhere.







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13.  Implementation Status

   [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






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   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 ISIS 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
   deploying the technology in different operational environments and
   network topologies.

14.1.  Verifying Forwarding on MRT Paths

   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
   period of time after a failure has been detected.  It is RECOMMENDED
   that an operator proactively monitor the MRT forwarding paths in



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   order to be certain that the paths will be able to carry FRR traffic
   when needed.  Therefore, an implementation SHOULD provide an operator
   with the ability to test MRT paths with OAM traffic.  For example,
   when MRT paths are realized using LDP labels distributed for
   topology-scoped FECs, an implementation can use the MPLS ping and
   traceroute as defined in [RFC4379] and extended in [RFC7307] for
   topology-scoped FECs.

14.2.  Traffic Capacity on Backup Paths

   During a fast-reroute event initiated by a PLR in response to a
   network failure, the flow of traffic in the network will generally
   not be identical to the flow of traffic after the IGP forwarding
   state has converged, taking the failure into account.  Therefore,
   even if a network has been engineered to have enough capacity on the
   appropriate links to carry all traffic after the IGP has converged
   after the failure, the network may still not have enough capacity on
   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 than might otherwise be expected.

   Note that there are two somewhat distinct aspects to this phenomenon.
   The first is that the path from the PLR to the destination during the
   fast-reroute event may be different from the path after the IGP
   converges.  In this case, any traffic for the destination that
   reaches the PLR during the fast-reroute event will follow a different
   path from the PLR to the destination than will be followed after IGP
   convergence.

   The second aspect is that the amount of traffic arriving at the PLR
   for affected destinations during the fast-reroute event may be larger
   than the amount of traffic arriving at the PLR for affected
   destinations after IGP convergence.  Immediately after a failure, any
   non-PLR routers that were sending traffic to the PLR before the
   failure will continue sending traffic to the PLR, and that traffic
   will be carried over backup paths from the PLR to the destinations.
   After IGP convergence, upstream non-PLR routers may direct some
   traffic away from the PLR.

   In order to reduce or eliminate the potential for transient traffic
   loss due to inadequate capacity during fast-reroute events, an
   operator can model the amount of traffic taking different paths
   during a fast-reroute event.  If it is determined that there is not
   enough capacity to support a given fast-reroute event, the operator
   can address the issue either by augmenting capacity on certain links
   or modifying the backup paths themselves.





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   The MRT Lowpoint algorithm produces a pair of diverse paths to each
   destination.  These paths are generated by following the directed
   links on a common GADAG.  MRT-FRR allows an operator to exclude a
   link from the MRT Island, and thus the GADAG, by advertising it as
   MRT-Ineligible.  Such a link will not be used on the MRT forwarding
   path for any destination.  Advertising links as MRT-Ineligible is the
   main tool provided by MRT-FRR for keeping backup traffic off of lower
   bandwidth links during fast-reroute events.

   Note that all of the backup paths produced by the MRT Lowpoint
   algorithm are closely tied to the common GADAG computed as part of
   that algorithm.  Therefore, it is generally not possible to modify a
   subset of paths without affecting other paths.  This precludes more
   fine-grained modification of individual backup paths when using only
   paths computed by the MRT Lowpoint algorithm.

   However, it may be desirable to allow an operator to use MRT-FRR
   alternates together with alternates provided by other FRR
   technologies.  A policy-based alternate selection process can allow
   an operator to select the best alternate from those provided by MRT
   and other FRR technologies.  As a concrete example, it may be
   desirable to implement a policy where a downstream LFA (if it exists
   for a given failure mode and destination) is preferred over a given
   MRT alternate.  This combination gives the operator the ability to
   affect where traffic flows during a fast-reroute event, while still
   producing backup paths that use no additional labels for LDP traffic
   and will not loop under multiple failures.  This and other choices of
   alternate selection policy can be evaluated in the context of their
   effect on fast-reroute traffic flow and available capacity, as well
   as other deployment considerations.

   Note that future documents may define MRT profiles in addition to the
   default profile defined here.  Different MRT profiles will generally
   produce alternate paths with different properties.  An implementation
   may allow an operator to use different MRT profiles instead of or in
   addition to the default profile.

14.3.  MRT IP Tunnel Loopback Address Management

   As described in Section 6.1.2, if an implementation uses IP tunneling
   as the mechanism to realize MRT forwarding paths, each node must
   advertise an MRT-Red and an MRT-Blue loopback address.  These IP
   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
   entries.  It is expected that operators will use existing tools and
   processes for managing infrastructure IP addresses to manage these
   additional MRT-related loopback addresses.




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14.4.  MRT-FRR in a Network with Degraded Connectivity

   Ideally, routers is a service provider network using MRT-FRR will be
   initially deployed in a 2-connected topology, allowing MRT-FRR to
   find completely diverse paths to all destinations.  However, a
   network can differ from an ideal 2-connected topology for many
   possible reasons, including network failures and planned maintenance
   events.

   MRT-FRR is designed to continue to function properly when network
   connectivity is degraded.  When a network contains cut-vertices or
   cut-links dividing the network into different 2-connected blocks,
   MRT-FRR will continue to provide completely diverse paths for
   destinations within the same block as the PLR.  For a destination in
   a different block from the PLR, the redundant paths created by MRT-
   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
   the topology.

   If a network becomes partitioned with one set of routers having no
   connectivity to another set of routers, MRT-FRR will function
   independently in each set of connected routers, providing redundant
   paths to destinations in same set of connected routers as a given
   PLR.

14.5.  Partial Deployment of MRT-FRR in a Network

   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
   partial deployment scenario.  Only routers that advertise support for
   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
   paths to all destinations with the MRT Island using maximally
   redundant trees all the way to those destinations.  For destinations
   outside of the MRT Island, MRT-FRR creates paths to the destination
   which use forwarding state created by MRT-FRR within the MRT Island
   and shortest path forwarding state outside of the MRT Island.  The
   paths created by MRT-FRR to non-Island destinations are guaranteed to
   be diverse within the MRT Island (if topologically possible).
   However, the part of the paths outside of the MRT Island may not be
   diverse.

15.  Acknowledgements

   The authors would like to thank Mike Shand for his valuable review
   and contributions.





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   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, and Stewart Bryant for
   their suggestions and review.

16.  IANA Considerations

   IANA is requested to create a registry entitled "MRT Profile
   Identifier Registry".  The range is 0 to 255.  The Default MRT
   Profile defined in this document has value 0.  Values 1-200 are
   allocated by Standards Action.  Values 201-220 are for
   experimentation.  Values 221-255 are for vendor private use.

17.  Security Considerations

   In general, MRT forwarding paths do not follow shortest paths.  The
   transit forwarding state corresponding to the MRT paths is created
   during normal operations (before a failure occurs).  Therefore, a
   malicious packet with an appropriate header injected into the network
   from a compromised location would be forwarded to a destination along
   a non-shortest path.  When this technology is deployed, a network
   security design should not rely on assumptions about potentially
   malicious traffic only following shortest paths.

   It should be noted that the creation of non-shortest forwarding paths
   is not unique to MRT.

   MRT-FRR requires that routers advertise information used in the
   formation of MRT backup paths.  While this document does not specify
   the protocol extensions used to advertise this information, we
   discuss security considerations related to the information itself.
   Injecting false MRT-related information could be used to direct some
   MRT backup paths over compromised transmission links.  Combined with
   the ability to generate network failures, this could be used to send
   traffic over compromised transmission links during a fast-reroute
   event.  In order to prevent this potential exploit, a receiving
   router needs to be able to authenticate MRT-related information that
   claims to have been advertised by another router.

18.  Contributors










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      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]
              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
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <http://www.rfc-editor.org/info/rfc5286>.







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19.2.  Informative References

   [EnyediThesis]
              Enyedi, G., "Novel Algorithms for IP Fast Reroute",
              Department of Telecommunications and Media Informatics,
              Budapest University of Technology and Economics Ph.D.
              Thesis, February 2011,
              <http://timon.tmit.bme.hu/theses/thesis_book.pdf>.

   [I-D.atlas-rtgwg-mrt-mc-arch]
              Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
              Envedi, "An Architecture for Multicast Protection Using
              Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
              arch-02 (work in progress), July 2013.

   [I-D.francois-rtgwg-segment-routing-ti-lfa]
              Francois, P., Filsfils, C., Bashandy, A., and B. Decraene,
              "Topology Independent Fast Reroute using Segment Routing",
              draft-francois-rtgwg-segment-routing-ti-lfa-00 (work in
              progress), August 2015.

   [I-D.ietf-rtgwg-lfa-manageability]
              Litkowski, S., Decraene, B., Filsfils, C., Raza, K.,
              Horneffer, M., and P. Sarkar, "Operational management of
              Loop Free Alternates", draft-ietf-rtgwg-lfa-
              manageability-11 (work in progress), June 2015.

   [I-D.ietf-rtgwg-rlfa-node-protection]
              Sarkar, P., Hegde, S., Bowers, C., Gredler, H., and S.
              Litkowski, "Remote-LFA Node Protection and Manageability",
              draft-ietf-rtgwg-rlfa-node-protection-05 (work in
              progress), December 2015.

   [LightweightNotVia]
              Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
              Fast ReRoute: Lightweight Not-Via without Additional
              Addresses", Proceedings of IEEE INFOCOM , 2009,
              <http://mycite.omikk.bme.hu/doc/71691.pdf>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <http://www.rfc-editor.org/info/rfc2328>.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.




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   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <http://www.rfc-editor.org/info/rfc5331>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <http://www.rfc-editor.org/info/rfc5340>.

   [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
              Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
              2009, <http://www.rfc-editor.org/info/rfc5443>.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, DOI 10.17487/RFC5714, January 2010,
              <http://www.rfc-editor.org/info/rfc5714>.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, DOI 10.17487/RFC5715, January
              2010, <http://www.rfc-editor.org/info/rfc5715>.

   [RFC6571]  Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
              B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
              <http://www.rfc-editor.org/info/rfc6571>.

   [RFC6976]  Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-Free
              Convergence Using the Ordered Forwarding Information Base
              (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
              2013, <http://www.rfc-editor.org/info/rfc6976>.

   [RFC6981]  Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
              and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
              DOI 10.17487/RFC6981, August 2013,
              <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.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <http://www.rfc-editor.org/info/rfc6987>.




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   [RFC7307]  Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
              King, "LDP Extensions for Multi-Topology", RFC 7307,
              DOI 10.17487/RFC7307, July 2014,
              <http://www.rfc-editor.org/info/rfc7307>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <http://www.rfc-editor.org/info/rfc7490>.

Appendix A.  General Issues with Area Abstraction

   When a multi-homed prefix is connected in two different areas, it may
   be impractical to protect them without adding the complexity of
   explicit tunneling.  This is also a problem for LFA and Remote-LFA.

          50
        |----[ASBR Y]---[B]---[ABR 2]---[C]      Backbone Area 0:
        |                                |           ABR 1, ABR 2, C, D
        |                                |
        |                                |       Area 20:  A, ASBR X
        |                                |
        p ---[ASBR X]---[A]---[ABR 1]---[D]      Area 10: B, ASBR Y
           5                                  p is a Type 1 AS-external


             Figure 5: AS external prefixes in different areas

   Consider the network in Figure 5 and assume there is a richer
   connective topology that isn't shown, where the same prefix is
   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
   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
   20.  This problem occurs because the routers, including the ABR, in
   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
   ASBR Y.  If the traffic is unlabeled or the appropriate MPLS labels
   are known, then explicit tunneling MAY be used as long as the
   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
   MRT alternate.








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Authors' Addresses

   Alia Atlas
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   USA

   Email: akatlas@juniper.net


   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   USA

   Email: cbowers@juniper.net


   Gabor Sandor Enyedi
   Ericsson
   Konyves Kalman krt 11.
   Budapest  1097
   Hungary

   Email: Gabor.Sandor.Enyedi@ericsson.com
























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