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Versions: (draft-bashandy-rtgwg-bgp-pic) 00 01 02 03 04 05

Network Working Group                                  A. Bashandy, Ed.
Internet Draft                                              C. Filsfils
Intended status: Informational                            Cisco Systems
Expires: November 2017                                     P. Mohapatra
                                                       Sproute Networks
                                                           May 25, 2017



                   BGP Prefix Independent Convergence
                     draft-ietf-rtgwg-bgp-pic-05.txt


Abstract

In the network comprising thousands of iBGP peers exchanging millions
of routes, many routes are reachable via more than one next-hop.
Given the large scaling targets, it is desirable to restore traffic
after failure in a time period that does not depend on the number of
BGP prefixes. In this document we proposed an architecture by which
traffic can be re-routed to ECMP or pre-calculated backup paths in a
timeframe that does not depend on the number of BGP prefixes. The
objective is achieved through organizing the forwarding data
structures in a hierarchical manner and sharing forwarding elements
among the maximum possible number of routes. The proposed technique
achieves prefix independent convergence while ensuring incremental
deployment, complete automation, and zero management and provisioning
effort. It is noteworthy to mention that the benefits of BGP-PIC are
hinged on the existence of more than one path whether as ECMP or
primary-backup.

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   other groups may also distribute working documents as Internet-
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Table of Contents

   1. Introduction...................................................3
      1.1. Conventions used in this document.........................4
      1.2. Terminology...............................................4
   2. Overview.......................................................6
      2.1. Dependency................................................6
         2.1.1. Hierarchical Hardware FIB............................6
         2.1.2. Availability of more than one primary or secondary BGP
         next-hops...................................................7
      2.2. BGP-PIC Illustration......................................7
   3. Constructing the Shared Hierarchical Forwarding Chain..........9
      3.1. Constructing the BGP-PIC forwarding Chain.................9
      3.2. Example: Primary-Backup Path Scenario....................10
   4. Forwarding Behavior...........................................11
   5. Handling Platforms with Limited Levels of Hierarchy...........12

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      5.1. Flattening the Forwarding Chain..........................12
      5.2. Example: Flattening a forwarding chain...................14
   6. Forwarding Chain Adjustment at a Failure......................21
      6.1. BGP-PIC core.............................................22
      6.2. BGP-PIC edge.............................................23
         6.2.1. Adjusting forwarding Chain in egress node failure...23
         6.2.2. Adjusting Forwarding Chain on PE-CE link Failure....23
      6.3. Handling Failures for Flattened Forwarding Chains........24
   7. Properties....................................................25
      7.1. Coverage.................................................25
         7.1.1. A remote failure on the path to a BGP next-hop......25
         7.1.2. A local failure on the path to a BGP next-hop.......25
         7.1.3. A remote iBGP next-hop fails........................26
         7.1.4. A local eBGP next-hop fails.........................26
      7.2. Performance..............................................26
      7.3. Automated................................................27
      7.4. Incremental Deployment...................................27
   8. Security Considerations.......................................27
   9. IANA Considerations...........................................27
   10. Conclusions..................................................27
   11. References...................................................28
      11.1. Normative References....................................28
      11.2. Informative References..................................28
   12. Acknowledgments..............................................29
   Appendix A. Perspective..........................................30

1. Introduction

   As a path vector protocol, BGP propagates reachability serially.
   Hence BGP convergence speed is limited by the time taken to
   serially propagate reachability information from the point of
   failure to the device that must re-converge. BGP speakers exchange
   reachability information about prefixes[2][3] and, for labeled
   address families, namely AFI/SAFI 1/4, 2/4, 1/128, and 2/128, an
   edge router assigns local labels to prefixes and associates the
   local label with each advertised prefix such as L3VPN [8], 6PE
   [9], and Softwire [7] using BGP label unicast technique[4]. A BGP
   speaker then applies the path selection steps to choose the best
   path. In modern networks, it is not uncommon to have a prefix
   reachable via multiple edge routers. In addition to proprietary
   techniques, multiple techniques have been proposed to allow for
   BGP to advertise more than one path for a given prefix
   [6][11][12], whether in the form of equal cost multipath or
   primary-backup. Another common and widely deployed scenario is
   L3VPN with multi-homed VPN sites with unique Route Distinguisher.
   It is advantageous to utilize the commonality among paths used by
   NLRIs to significantly improve convergence in case of topology
   modifications.



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   This document proposes a hierarchical and shared forwarding chain
   organization that allows traffic to be restored to pre-calculated
   alternative equal cost primary path or backup path in a time
   period that does not depend on the number of BGP prefixes. The
   technique relies on internal router behavior that is completely
   transparent to the operator and can be incrementally deployed and
   enabled with zero operator intervention.

1.1. Conventions used in this document

   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 RFC-2119
   [1].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to
   be interpreted as carrying RFC-2119 significance.

1.2. Terminology

   This section defines the terms used in this document. For ease of
   use, we will use terms similar to those used by L3VPN [8]

   o  BGP prefix: A prefix P/m (of any AFI/SAFI) that a BGP speaker
      has a path for.

   o  IGP prefix: A prefix P/m (of any AFI/SAFI) that is learnt via
      an Interior Gateway Protocol, such as OSPF and ISIS, has a path
      for. The prefix may be learnt directly through the IGP or
      redistributed from other protocol(s)

   o  CE: An external router through which an egress PE can reach a
      prefix P/m.

   o  Ingress PE, "iPE": A BGP speaker that learns about a prefix
      through a IBGP peer and chooses an egress PE as the next-hop for
      the prefix.

   o  Path: The next-hop in a sequence of nodes starting from the
      current node and ending with the destination node or network
      identified by the prefix. The nodes may not be directly
      connected.

   o  Recursive path: A path consisting only of the IP address of the
      next-hop without the outgoing interface. Subsequent lookups are
      necessary to determine the outgoing interface and a directly
      connected next-hop



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   o  Non-recursive path: A path consisting of the IP address of a
      directly connected next-hop and outgoing interface

   o  Primary path: A recursive or non-recursive path that can be
      used all the time as long as a walk starting from this path can
      end to an adjacency. A prefix can have more than one primary
      path

   o  Backup path: A recursive or non-recursive path that can be used
      only after some or all primary paths become unreachable

   o  Leaf: A container data structure for a prefix or local label.
      Alternatively, it is the data structure that contains prefix
      specific information.

   o  IP leaf: The leaf corresponding to an IPv4 or IPv6 prefix

   o  Label leaf. The leaf corresponding to a locally allocated label
      such as the VPN label on an egress PE [8].

   o  Pathlist: An array of paths used by one or more prefix to forward
      traffic to destination(s) covered by a IP prefix. Each path in
      the pathlist carries its "path-index" that identifies its
      position in the array of paths. "). In general, the value of the
      "path-index" stored in path may not necessarily has the same
      value of the location of the path in the pathlist. For example
      the 3rd path may carry path-index value of 1

   o  A pathlist may contain a mix of primary and backup paths

   o  OutLabel-List: Each labeled prefix is associated with an
      OutLabel-List. The OutLabel-List is an array of one or more
      outgoing labels and/or label actions where each label or label
      action has 1-to-1 correspondence to a path in the pathlist.
      Label actions are: push the label, pop the label, swap the
      incoming label with the label in the Outlabel-Array entry, or
      don't push anything at all in case of "unlabeled". The prefix
      may be an IGP or BGP prefix

   o  Adjacency: The layer 2 encapsulation leading to the layer 3
      directly connected next-hop

   o  Dependency: An object X is said to be a dependent or child of
      object Y if there is at least one forwarding chain where the
      forwarding engine must visits the object X before visiting the
      object Y in order to forward a packet. Note that if object X is
      a child of object Y, then Y cannot be deleted unless object X
      is no longer a dependent/child of object Y



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   o  Route: A prefix with one or more paths associated with it.
      Hence the minimum set of objects needed to construct a route is
      a leaf and a pathlist.



2. Overview

   The idea of BGP-PIC is based on two pillars

   o  A shared hierarchical forwarding Chain: It is not uncommon to see
      multiple destinations are reachable via the same list of next-
      hops. Instead of having a separate list of next-hops for each
      destination, all destinations sharing the same list of next-hops
      can point to a single copy of this list thereby allowing fast
      convergence by making changes to a single shared list of next-
      hops rather than possibly a large number of destinations. Because
      paths in a pathlist may be recursive, a hierarchy is formed
      between pathlist and the resolving prefix whereby the pathlist
      depends on the resolving prefix.

   o  A forwarding plane that supports multiple levels of indirection:
      A forwarding that starts with a destination and ends with an
      outgoing interface is not a simple flat structure. Instead a
      forwarding entry is constructed via multiple levels of
      dependency. A BGP NLRI uses a recursive next-hop, which in turn
      resolves via an IGP next-hop, which in turn resolves via an
      adjacency consisting of one or more outgoing interface(s) and
      next-hop(s).

   Designing a forwarding plane that constructs multi-level forwarding
   chains with maximal sharing of forwarding objects allows rerouting a
   large number of destinations by modifying a small number of objects
   thereby achieving convergence in a time frame that does not depend
   on the number of destinations. For example, if the IGP prefix that
   resolves a recursive next-hop is updated there is no need to update
   the possibly large number of BGP NLRIs that use this recursive next-
   hop.

2.1. Dependency

   This section describes the required functionality in the forwarding
   and control planes to support BGP-PIC described in this document

2.1.1. Hierarchical Hardware FIB

   BGP PIC requires a hierarchical hardware FIB support: for each BGP
   forwarded packet, a BGP leaf is looked up, then a BGP Pathlist is
   consulted, then an IGP Pathlist, then an Adjacency.


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   An alternative method consists in "flattening" the dependencies when
   programming the BGP destinations into HW FIB resulting in
   potentially eliminating both the BGP Path-List and IGP Path-List
   consultation. Such an approach decreases the number of memory
   lookup's per forwarding operation at the expense of HW FIB memory
   increase (flattening means less sharing hence duplication), loss of
   ECMP properties (flattening means less pathlist entropy) and loss of
   BGP PIC properties.

2.1.2. Availability of more than one primary or secondary BGP next-hops

   When the primary BGP next-hop fails, BGP PIC depends on the
   availability of a pre-computed and pre-installed secondary BGP next-
   hop in the BGP Pathlist.

   The existence of a secondary next-hop is clear for the following
   reason: a service caring for network availability will require two
   disjoint network connections hence two BGP next-hops.

   The BGP distribution of the secondary next-hop is available thanks
   to the following BGP mechanisms: Add-Path [11], BGP Best-External
   [6], diverse path [12], and the frequent use in VPN deployments of
   different VPN RD's per PE. It is noteworthy to mention that the
   availability of another BGP path does not mean that all failure
   scenarios can be covered by simply forwarding traffic to the
   available secondary path. The discussion of how to cover various
   failure scenarios is beyond the scope of this document

2.2. BGP-PIC Illustration

   To illustrate the two pillars above as well as the platform
   dependency, we will use an example of a simple multihomed L3VPN [8]
   prefix in a BGP-free core running LDP [5] or segment routing over
   MPLS forwarding plane [14].

    +--------------------------------+
    |                                |
    |                               ePE2 (IGP-IP1 192.0.2.1, Loopback)
    |                                |  \
    |                                |   \
    |                                |    \
   iPE                               |    CE....VRF "Blue", ASnum 65000
    |                                |    /    (VPN-IP1 11.1.1.0/24)
    |                                |   /     (VPN-IP2 11.1.2.0/24)
    |   LDP/Segment-Routing Core     |  /
    |                               ePE1 (IGP-IP2 192.0.2.2, Loopback)
    |                                |
    +--------------------------------+
             Figure 1 VPN prefix reachable via multiple PEs


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   Referring to Figure 1, suppose the iPE (the ingress PE) receives
   NLRIs for the VPN prefixes VPN-IP1 and VPN-IP2 from two egress PEs,
   ePE1 and ePE2 with next-hop BGP-NH1 and BGP-NH2, respectively.
   Assume that ePE1 advertise the VPN labels VPN-L11 and VPN-L12 while
   ePE2 advertise the VPN labels VPN-L21 and VPN-L22 for VPN-IP1 and
   VPN-IP2, respectively. Suppose that BGP-NH1 and BGP-NH2 are resolved
   via the IGP prefixes IGP-IP1 and IGP-P2, where each happen to have 2
   ECMP paths with IGP-NH1 and IGP-NH2 reachable via the interfaces I1
   and I2, respectively. Suppose that local labels (whether LDP [5] or
   segment routing [14]) on the downstream LSRs for IGP-IP1 are IGP-L11
   and IGP-L12 while for IGP-P2 are IGP-L21 and IGP-L22. As such, the
   routing table at iPE is as follows:

         65000:11.1.1.0/24
            via ePE1 (192.0.2.1), VPN Label: VPN-L11
            via ePE2 (192.0.2.2), VPN Label: VPN-L21

         65000:11.1.2.0/24
            via ePE1 (192.0.2.1), VPN Label: VPN-L12
            via ePE2 (192.0.2.2), VPN Label: VPN-L22

         192.0.2.1/32
            via Core, Label:  IGP-L11
            via Core, Label:  IGP-L12

         192.0.2.2/32
            via Core, Label:  IGP-L21
            via Core, Label:  IGP-L22


   Based on the above routing table, a hierarchical forwarding chain
   can be constructed as shown in Figure 2.

   IP Leaf:    Pathlist:    IP Leaf:           Pathlist:
   --------  +-------+     --------          +----------+
   VPN-IP1-->|BGP-NH1|-->IGP-IP1(BGP NH1)--->|IGP NH1,I1|--->Adjacency1
     |       |BGP-NH2|-->....      |         |IGP NH2,I2|--->Adjacency2
     |       +-------+             |         +----------+
     |                             |
     |                             |
     v                             v
   OutLabel-List:                OutLabel-List:
   +----------------------+      +----------------------+
   |VPN-L11 (VPN-IP1, NH1)|      |IGP-L11 (IGP-IP1, NH1)|
   |VPN-L12 (VPN-IP1, NH2)|      |IGP-L12 (IGP-IP1, NH2)|
   +----------------------+      +----------------------+

           Figure 2 Shared Hierarchical Forwarding Chain at iPE



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   The forwarding chain depicted in Figure 2 illustrates the first
   pillar, which is sharing and hierarchy. We can see that the BGP
   pathlist consisting of BGP-NH1 and BGP-NH2 is shared by all NLRIs
   reachable via ePE1 and ePE2. As such, it is possible to make changes
   to the pathlist without having to make changes to the NLRIs. For
   example, if BGP-NH2 becomes unreachable, there is no need to modify
   any of the possibly large number of NLRIs. Instead only the shared
   pathlist needs to be modified. Likewise, due to the hierarchical
   structure of the forwarding chain, it is possible to make
   modifications to the IGP routes without having to make any changes
   to the BGP NLRIs. For example, if the interface "I2" goes down, only
   the shared IGP pathlist needs to be updated, but none of the IGP
   prefixes sharing the IGP pathlist nor the BGP NLRIs using the IGP
   prefixes for resolution need to be modified.

   Figure 2 can also be used to illustrate the second BGP-PIC pillar.
   Having a deep forwarding chain such as the one illustrated in Figure
   2 requires a forwarding plane that is capable of accessing multiple
   levels of indirection in order to calculate the outgoing
   interface(s) and next-hops(s). While a deeper forwarding chain
   minimizes the re-convergence time on topology change, there will
   always exist platforms with limited capabilities and hence imposing
   a limit on the depth of the forwarding chain. Section 5 describes
   how to gracefully trade off convergence speed with the number of
   hierarchical levels to support platforms with different
   capabilities.

3. Constructing the Shared Hierarchical Forwarding Chain

   Constructing the forwarding chain is an application of the two
   pillars described in Section 2. This section describes how to
   construct the forwarding chain in hierarchical shared manner

3.1. Constructing the BGP-PIC forwarding Chain

   The whole process starts when BGP downloads a prefix to FIB. The
   prefix contains one or more outgoing paths. For certain labeled
   prefixes, such as VPN [8] prefixes, each path may be associated with
   an outgoing label and the prefix itself may be assigned a local
   label. The list of outgoing paths defines a pathlist. If such
   pathlist does not already exist, then FIB creates a new pathlist,
   otherwise the existing pathlist is used. The BGP prefix is added as
   a dependent of the pathlist.

   The previous step constructs the upper part of the hierarchical
   forwarding chain. The forwarding chain is completed by resolving the
   paths of the pathlist. A BGP path usually consists of a next-hop.
   The next-hop is resolved by finding a matching IGP prefix.


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   The end result is a hierarchical shared forwarding chain where the
   BGP pathlist is shared by all BGP prefixes that use the same list of
   paths and the IGP prefix is shared by all pathlists that have a path
   resolving via that IGP prefix. It is noteworthy to mention that the
   forwarding chain is constructed without any operator intervention at
   all.

   The remainder of this section goes over an example to illustrate the
   applicability of BGP-PIC in a primary-backup path scenario.

3.2. Example: Primary-Backup Path Scenario

   Consider the egress PE ePE1 in the case of the multi-homed VPN
   prefixes in the BGP-free core depicted in Figure 1. Suppose ePE1
   determines that the primary path is the external path but the backup
   path is the iBGP path to the other PE ePE2 with next-hop BGP-NH2.
   ePE2 constructs the forwarding chain depicted in Figure 3. We are
   only showing a single VPN prefix for simplicity. But all prefixes
   that are multihomed to ePE1 and ePE2 share the BGP pathlist.


                    BGP OutLabel Array
     VPN-L11            +---------+
   (Label-leaf)---+---->|Unlabeled|
                  |     +---------+
                  |     | VPN-L21 |
                  |     | (swap)  |
                  |     +---------+
                  |
                  |                    BGP Pathlist
                  |                   +------------+    Connected route
                  |                   |   CE-NH    |------>(to the CE)
                  |                   |path-index=0|
                  |                   +------------+
                  |                   |  VPN-NH2   |
     VPN-IP1 -----+------------------>|  (backup)  |------>IGP Leaf
   (IP prefix leaf)                   |path-index=1|    (Towards ePE2)
        |                             +------------+
        |
        |           BGP OutLabel Array
        |              +---------+
        +------------->|Unlabeled|
                       +---------+
                       | VPN-L21 |
                       | (push)  |
                       +---------+

   Figure 3 : VPN Prefix Forwarding Chain with eiBGP paths on egress PE



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   The example depicted in Figure 3 differs from the example in Figure
   2 in two main aspects. First, as long as the primary path towards
   the CE (external path) is useable, it will be the only path used for
   forwarding while the OutLabel-List contains both the unlabeled label
   (primary path) and the VPN label (backup path) advertised by the
   backup path ePE2. The second aspect is presence of the label leaf
   corresponding to the VPN prefix. This label leaf is used to match
   VPN traffic arriving from the core. Note that the label leaf shares
   the pathlist with the IP prefix.


4. Forwarding Behavior

   This section explains how the forwarding plane uses the hierarchical
   shared forwarding chain to forward a packet.

   When a packet arrives at a router, it matches a leaf. A labeled
   packet matches a label leaf while an IP packet matches an IP prefix
   leaf. The forwarding engines walks the forwarding chain starting
   from the leaf until the walk terminates on an adjacency. Thus when a
   packet arrives, the chain is walked as follows:

   1. Lookup the leaf based on the destination address or the label at
      the top of the packet

   2. Retrieve the parent pathlist of the leaf

   3. Pick the outgoing path "Pi" from the list of resolved paths in
      the pathlist. The method by which the outgoing path is picked is
      beyond the scope of this document (e.g. flow-preserving hash
      exploiting entropy within the MPLS stack and IP header). Let the
      "path-index" of the outgoing path "Pi" be "j".

   4. If the prefix is labeled, use the "path-index" "j" to retrieve
      the jth label "Lj" stored the jth entry in the OutLabel-List and
      apply the label action of the label on the packet (e.g. for VPN
      label on the ingress PE, the label action is "push"). As
      mentioned in Section 1.2, the value of the "path-index" stored
      in path may not necessarily be the same value of the location of
      the path in the pathlist.

   5. Move to the parent of the chosen path "Pi"

   6. If the chosen path "Pi" is recursive, move to its parent prefix
      and go to step 2

   7. If the chosen path is non-recursive move to its parent adjacency.
      Otherwise go to the next step.



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   8. Encapsulate the packet in the layer string specified by the
      adjacency and send the packet out.

   Let's apply the above forwarding steps to the forwarding chain
   depicted in Figure 2 in Section 2. Suppose a packet arrives at
   ingress PE iPE from an external neighbor. Assume the packet matches
   the VPN prefix VPN-IP1. While walking the forwarding chain, the
   forwarding engine applies a hashing algorithm to choose the path and
   the hashing at the BGP level yields path 0 while the hashing at the
   IGP level yields path 1. In that case, the packet will be sent out
   of interface I2 with the label stack "IGP-L12,VPN-L11".

5. Handling Platforms with Limited Levels of Hierarchy

   This section describes the construction of the forwarding chain if a
   platform does not support the number of recursion levels required to
   resolve the NLRIs. There are two main design objectives

   o  Being able to reduce the number of hierarchical levels from any
      arbitrary value to a smaller arbitrary value that can be
      supported by the forwarding engine

   o  Minimal modifications to the forwarding algorithm due to such
      reduction.

5.1. Flattening the Forwarding Chain

   Let's consider a pathlist associated with the leaf "R1" consisting
   of the list of paths <P1, P2,..., Pn>. Assume that the leaf "R1" has
   an Outlabel-list <L1, L2,..., Ln>. Suppose the path Pi is a
   recursive path that resolves via a prefix represented by the leaf
   "R2". The leaf "R2" itself is pointing to a pathlist consisting of
   the paths <Q1, Q2,..., Qm>

   If the platform supports the number of hierarchy levels of the
   forwarding chain, then a packet that uses the path "Pi" will be
   forwarded as follows:

   1. The forwarding engine is now at leaf "R1"

   2. So it moves to its parent pathlist, which contains the list <P1,
      P2,..., Pn>.

   3. The forwarding engine applies a hashing algorithm and picks the
      path "Pi". So now the forwarding engine is at the path "Pi"

   4. The forwarding engine retrieves the label "Li" from the outlabel-
      list attached to the leaf "R1" and applies the label action

   5. The path "Pi" uses the leaf "R2"

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   6. The forwarding engine walks forward to the leaf "R2" for
      resolution

   7. The forwarding plane performs a hash to pick a path among the
      pathlist of the leaf "R2", which is <Q1, Q2,..., Qm>

   8. Suppose the forwarding engine picks the path "Qj"

   9. Now the forwarding engine continues the walk to the parent of
      "Qj"

   Suppose the platform cannot support the number of hierarchy levels
   in the forwarding chain. FIB needs to reduce the number of hierarchy
   levels. The idea of reducing the number of hierarchy levels is to
   "flatten" two chain levels into a single level. The "flattening"
   steps are as follows

   1. FIB wants to reduce the number of levels used by "Pi" by 1

   2. FIB walks to the parent of "Pi", which is the leaf "R2"

   3. FIB extracts the parent pathlist of the leaf "R2", which is <Q1,
      Q2,..., Qm>

   4. FIB also extracts the OutLabel-list(R2) associated with the leaf
      "R2". Remember that OutLabel-list(R2) = <L1, L2,..., Lm>

   5. FIB replaces the path "Pi", with the list of paths <Q1, Q2,...,
      Qm>

   6. Hence the path list <P1, P2,..., Pn> now becomes "<P1, P2,...,Pi-
      1, Q1, Q2,..., Qm, Pi+1, Pn>

   7. The path index stored inside the locations "Q1", "Q2", ..., "Qm"
      must all be "i" because the index "i" refers to the label "Li"
      associated with leaf "R1"

   8. FIB attaches an OutLabel-list with the new pathlist as follows:
      <Unlabeled,..., Unlabeled, L1, L2,..., Lm, Unlabeled, ...,
      Unlabeled>. The size of the label list associated with the
      flattened pathlist equals the size of the pathlist. Hence there
      is a 1-1 mapping between every path in the "flattened" pathlist
      and the OutLabel-list associated with it.

   It is noteworthy to mention that the labels in the outlabel-list
   associated with the "flattened" pathlist may be stored in the same
   memory location as the path itself to avoid additional memory
   access. But that is an implementation detail that is beyond the
   scope of this document.


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   The same steps can be applied to all paths in the pathlist <P1,
   P2,..., Pn> so that all paths are "flattened" thereby reducing the
   number of hierarchical levels by one. Note that that "flattening" a
   pathlist pulls in all paths of the parent paths, a desired feature
   to utilize all ECMP/UCMP paths at all levels. A platform that has a
   limit on the number of paths in a pathlist for any given leaf may
   choose to reduce the number paths using methods that are beyond the
   scope of this document.

   The steps can be recursively applied to other paths at the same
   levels or other levels to recursively reduce the number of
   hierarchical levels to an arbitrary value so as to accommodate the
   capability of the forwarding engine.

   Because a flattened pathlist may have an associated OutLabel-list
   the forwarding behavior has to be slightly modified. The
   modification is done by adding the following step right after step 4
   in Section 4.

   5. If there is an OutLabel-list associated with the pathlist, then
      if the path "Pi" is chosen by the hashing algorithm, retrieve the
      label at location "i" in that OutLabel-list and apply the label
      action of that label on the packet

   In the next subsection, we apply the steps in this subsection to a
   sample scenario.

5.2. Example: Flattening a forwarding chain

   This example uses a case of inter-AS option C [8] where there are 3
   levels of hierarchy. Figure 4 illustrates the sample topology. To
   force 3 levels of hierarchy, the ASBRs on the ingress domain (domain
   1) advertise the core routers of the egress domain (domain 2) to the
   ingress PE (iPE) via BGP-LU [4] instead of redistributing them into
   the IGP of domain 1. The end result is that the ingress PE (iPE) has
   2 levels of recursion for the VPN prefix VPN-IP1 and VPN2-IP2.















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        Domain 1                  Domain 2
    +-------------+            +-------------+
    |             |            |             |
    | LDP/SR Core |            | LDP/SR core |
    |             |            |             |
    |     (192.0.1.1)          |             |
    |         ASBR11---------ASBR21........ePE1(192.0.2.1)
    |             |  \      /  |   .      .  |\
    |             |   \    /   |    .    .   | \
    |             |    \  /    |     .  .    |  \
    |             |     \/     |      ..     |   \VPN-IP1 (11.1.1.0/24)
    |             |     /\     |      . .    |   /VRF "Blue" ASn: 65000
    |             |    /  \    |     .   .   |  /
    |             |   /    \   |    .     .  | /
    |             |  /      \  |   .       . |/
   iPE        ASBR12---------ASBR22........ePE2 (192.0.2.2)
    |     (192.0.1.2)          |             |\
    |             |            |             | \
    |             |            |             |  \
    |             |            |             |   \VRF "Blue" ASn: 65000
    |             |            |             |   /VPN-IP2 (11.1.2.0/24)
    |             |            |             |  /
    |             |            |             | /
    |             |            |             |/
    |         ASBR13---------ASBR23........ePE3(192.0.2.3)
    |     (192.0.1.3)          |             |
    |             |            |             |
    |             |            |             |
    +-------------+            +-------------+
     <============  <=========  <============
    Advertise ePEx   Advertise   Redistribute
    Using iBGP-LU    ePEx Using    IGP into
                       eBGP-LU        BGP

               Figure 4 : Sample 3-level hierarchy topology



   We will make the following assumptions about connectivity

   o  In "domain 2", both ASBR21 and ASBR22 can reach both ePE1 and
      ePE2 using the same distance

   o  In "domain 2", only ASBR23 can reach ePE3

   o  In "domain 1", iPE (the ingress PE) can reach ASBR11, ASBR12, and
      ASBR13 via IGP using the same distance.

   We will make the following assumptions about the labels

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   o  The VPN labels advertised by ePE1 and ePE2 for prefix VPN-IP1 are
      VPN-L11 and VPN-L21, respectively

   o  The VPN labels advertised by ePE2 and ePE3 for prefix VPN-IP2 are
      VPN-L22 and VPN-L32, respectively

   o  The labels advertised by ASBR11 to iPE using BGP-LU [4] for the
      egress PEs ePE1 and ePE2 are LASBR11(ePE1) and LASBR11(ePE2),
      respectively.

   o  The labels advertised by ASBR12 to iPE using BGP-LU [4] for the
      egress PEs ePE1 and ePE2 are LASBR12(ePE1) and LASBR12(ePE2),
      respectively

   o  The label advertised by ASBR11 to iPE using BGP-LU [4] for the
      egress PE ePE3 is LASBR13(ePE3)

   o  The IGP labels advertised by the next hops directly connected to
      iPE towards ASBR11, ASBR12, and ASBR13 in the core of domain 1
      are IGP-L11, IGP-L12, and IGP-L13, respectively.

   Based on these connectivity assumptions and the topology in Figure
   4, the routing table on iPE is

         65000:11.1.1.0/24
            via ePE1 (192.0.2.1), VPN Label: VPN-L11
            via ePE2 (192.0.2.2), VPN Label: VPN-L21
         65000:11.1.2.0/24
            via ePE1 (192.0.2.2), VPN Label: VPN-L22
            via ePE2 (192.0.2.3), VPN Label: VPN-L23

         192.0.2.1/32 (ePE1)
            Via ASBR11, BGP-LU Label: LASBR11(ePE1)
            Via ASBR12, BGP-LU Label: LASBR12(ePE1)
         192.0.2.2/32 (ePE2)
            Via ASBR11, BGP-LU Label: LASBR11(ePE2)
            Via ASBR12, BGP-LU Label: LASBR12(ePE2)
         192.0.2.3/32 (ePE3)
            Via ASBR13, BGP-LU Label: LASBR13(ePE3)

         192.0.1.1/32 (ASBR11)
            via Core, Label:  IGP-L11
         192.0.1.2/32 (ASBR12)
            via Core, Label:  IGP-L12
         192.0.1.3/32 (ASBR13)
            via Core, Label:  IGP-L13





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   The diagram in Figure 5 illustrates the forwarding chain in iPE
   assuming that the forwarding hardware in iPE supports 3 levels of
   hierarchy. The leaves corresponding to the ABSRs on domain 1
   (ASBR11, ASBR12, and ASBR13) are at the bottom of the hierarchy.
   There are few important points:

   o  Because the hardware supports the required depth of hierarchy,
      the sizes of a pathlist equal the size of the label list
      associated with the leaves using this pathlist

   o  The index inside the pathlist entry indicates the label that will
      be picked from the Outlabel-List associated with the child leaf
      if that path is chosen by the forwarding engine hashing function.






































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   Outlabel-List                                      Outlabel-List
     For VPN-IP1                                         For VPN-IP2
   +------------+    +--------+           +-------+   +------------+
   |  VPN-L11   |<---| VPN-IP1|           |VPN-IP2|-->|  VPN-L22   |
   +------------+    +---+----+           +---+---+   +------------+
   |  VPN-L21   |        |                    |       |  VPN-L32   |
   +------------+        |                    |       +------------+
                         |                    |
                         V                    V
                    +---+---+            +---+---+
                    | 0 | 1 |            | 0 | 1 |
                    +-|-+-\-+            +-/-+-\-+
                      |    \              /     \
                      |     \            /       \
                      |      \          /         \
                      |       \        /           \
                      v        \      /             \
                 +-----+       +-----+             +-----+
            +----+ ePE1|       |ePE2 +-----+       | ePE3+-----+
            |    +--+--+       +-----+     |       +--+--+     |
            v       |            /         v          |        v
   +-------------+  |           /   +-------------+   | +-------------+
   |LASBR11(ePE1)|  |          /    |LASBR11(ePE2)|   | |LASBR13(ePE3)|
   +-------------+  |         /     +-------------+   | +-------------+
   |LASBR12(ePE1)|  |        /      |LASBR12(ePE2)|   | Outlabel-List
   +-------------+  |       /       +-------------+   |    For ePE3
   Outlabel-List    |      /        Outlabel-List     |
       For ePE1     |     /           For ePE2        |
                    |    /                            |
                    |   /                             |
                    |  /                              |
                    v /                               v
                +---+---+  Shared Pathlist          +---+  Pathlist
                | 0 | 1 | For ePE1 and ePE2         | 0 |  For ePE3
                +-|-+-\-+                           +-|-+
                  |    \                              |
                  |     \                             |
                  |      \                            |
                  |       \                           |
                  v        \                          v
               +------+    +------+               +------+
           +---+ASBR11|    |ASBR12+--+            |ASBR13+---+
           |   +------+    +------+  |            +------+   |
           v                         v                       v
      +-------+                  +-------+              +-------+
      |IGP-L11|                  |IGP-L12|              |IGP-L13|
      +-------+                  +-------+              +-------+

       Figure 5 : Forwarding Chain for hardware supporting 3 Levels


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   Now suppose the hardware on iPE (the ingress PE) supports 2 levels
   of hierarchy only. In that case, the 3-levels forwarding chain in
   Figure 5 needs to be "flattened" into 2 levels only.


   Outlabel-List                                  Outlabel-List
     For VPN-IP1                                    For VPN-IP2
   +------------+    +-------+      +-------+     +------------+
   |  VPN-L11   |<---|VPN-IP1|      | VPN-IP2|--->|  VPN-L22   |
   +------------+    +---+---+      +---+---+     +------------+
   |  VPN-L21   |        |              |         |  VPN-L32   |
   +------------+        |              |         +------------+
                         |              |
                         |              |
                         |              |
          Flattened      |              |  Flattened
          pathlist       V              V   pathlist
                    +===+===+        +===+===+===+     +=============+
           +--------+ 0 | 1 |        | 0 | 0 | 1 +---->|LASBR11(ePE2)|
           |        +=|=+=\=+        +=/=+=/=+=\=+     +=============+
           v          |    \          /   /     \      |LASBR12(ePE2)|
    +=============+   |     \  +-----+   /       \     +=============+
    |LASBR11(ePE1)|   |      \/         /         \    |LASBR13(ePE3)|
    +=============+   |      /\        /           \   +=============+
    |LASBR12(ePE1)|   |     /  \      /             \
    +=============+   |    /    \    /               \
                      |   /      \  /                 \
                      |  /       +  +                  \
                      |  +       |  |                   \
                      |  |       |  |                    \
                      v  v       v  v                     \
                    +------+    +------+              +------+
               +----|ASBR11|    |ASBR12+---+          |ASBR13+---+
               |    +------+    +------+   |          +------+   |
               v                           v                     v
           +-------+                  +-------+              +-------+
           |IGP-L11|                  |IGP-L12|              |IGP-L13|
           +-------+                  +-------+              +-------+

     Figure 6 : Flattening 3 levels to 2 levels of Hierarchy on iPE



   Figure 6 represents one way to "flatten" a 3 levels hierarchy into
   two levels. There are few important points:





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   o  As mentioned in Section 5.1 a flattened pathlist may have label
      lists associated with them. The size of the label list associated
      with a flattened pathlist equals the size of the pathlist. Hence
      it is possible that an implementation includes these label lists
      in the flattened pathlist itself

   o  Again as mentioned in Section 5.1, the size of a flattened
      pathlist may not be equal to the size of the OutLabel-lists of
      leaves using the flattened pathlist. So the indices inside a
      flattened pathlist still indicate the label index in the
      Outlabel-Lists of the leaves using that pathlist. Because the
      size of the flattened pathlist may be different from the size of
      the OutLabel-lists of the leaves, the indices may be repeated.

   o  Let's take a look at the flattened pathlist used by the prefix
      "VPN-IP2", The pathlist associated with the prefix "VPN-IP2" has
      three entries.

       o The first and second entry have index "0". This is because
         both entries correspond to ePE2. Hence when hashing performed
         by the forwarding engine results in using first or the second
         entry in the pathlist, the forwarding engine will pick the
         correct VPN label "VPN-L22", which is the label advertised by
         ePE2 for the prefix "VPN-IP2"

       o The third entry has the index "1". This is because the third
         entry corresponds to ePE3. Hence when the hashing is performed
         by the forwarding engine results in using the third entry in
         the flattened pathlist, the forwarding engine will pick the
         correct VPN label "VPN-L32", which is the label advertised by
         "ePE3" for the prefix "VPN-IP2"

   Now let's try and apply the forwarding steps in Section 4 together
   with the additional step in Section 5.1 to the flattened forwarding
   chain illustrated in Figure 6.

   o  Suppose a packet arrives at "iPE" and matches the VPN prefix
      "VPN-IP2"

   o  The forwarding engine walks to the parent of the "VPN-IP2", which
      is the flattened pathlist and applies a hashing algorithm to pick
      a path

   o  Suppose the hashing by the forwarding engine picks the second
      path in the flattened pathlist associated with the leaf "VPN-
      IP2".

   o  Because the second path has the index "0", the label "VPN-L22" is
      pushed on the packet


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   o  Next the forwarding engine picks the second label from the
      Outlabel-Array associated with the flattened pathlist. Hence the
      next label that is pushed is "LASBR12(ePE2)"

   o  The forwarding engine now moves to the parent of the flattened
      pathlist corresponding to the second path. The parent is the IGP
      label leaf corresponding to "ASBR12"

   o  So the packet is forwarded towards the ASBR "ASBR12" and the IGP
      label at the top will be "L12"

   Based on the above steps, a packet arriving at iPE and destined to
   the prefix VPN-L22 reaches its destination as follows

   o  iPE sends the packet along the shortest path towards  ASBR12 with
      the following label stack starting from the top: {L12,
      LASBR12(ePE2), VPN-L22}.

   o  The penultimate hop of ASBR12 pops the top label "L12". Hence the
      packet arrives at ASBR12 with the label stack {LASBR12(ePE2),
      VPN-L22} where "LASBR12(ePE2)" is the top label.

   o  ASBR12 swaps "LASBR12(ePE2)" with the label "LASBR22(ePE2)",
      which is the label advertised by ASBR22 for the ePE2 (the egress
      PE).

   o  ASBR22 receives the packet with "LASBR22(ePE2)" at the top.

   o  Hence ASBR22 swaps "LASBR22(ePE2)" with the IGP label for ePE2
      advertised by the next-hop towards ePE2 in domain 2, and sends
      the packet along the shortest path towards ePE2.

   o  The penultimate hop of ePE2 pops the top label. Hence ePE2
      receives the packet with the top label VPN-L22 at the top

   o  ePE2 pops "VPN-L22" and sends the packet as a pure IP packet
      towards the destination VPN-IP2.



6. Forwarding Chain Adjustment at a Failure

   The hierarchical and shared structure of the forwarding chain
   explained in the previous section allows modifying a small number of
   forwarding chain objects to re-route traffic to a pre-calculated
   equal-cost or backup path without the need to modify the possibly
   very large number of BGP prefixes. In this section, we go over
   various core and edge failure scenarios to illustrate how FIB
   manager can utilize the forwarding chain structure to achieve BGP
   prefix independent convergence.

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6.1. BGP-PIC core

   This section describes the adjustments to the forwarding chain when
   a core link or node fails but the BGP next-hop remains reachable.

   There are two case: remote link failure and attached link failure.
   Node failures are treated as link failures.

   When a remote link or node fails, IGP on the ingress PE receives
   advertisement indicating a topology change so IGP re-converges to
   either find a new next-hop and/or outgoing interface or remove the
   path completely from the IGP prefix used to resolve BGP next-hops.
   IGP and/or LDP download the modified IGP leaves with modified
   outgoing labels for labeled core.

   When a local link fails, FIB manager detects the failure almost
   immediately. The FIB manager marks the impacted path(s) as unusable
   so that only useable paths are used to forward packets. Hence only
   IGP pathlists with paths using the failed local link need to be
   modified. All other pathlists are not impacted. Note that in this
   particular case there is actually no need even to backwalk to IGP
   leaves to adjust the OutLabel-Lists because FIB can rely on the
   path-index stored in the useable paths in the pathlist to pick the
   right label.

   It is noteworthy to mention that because FIB manager modifies the
   forwarding chain starting from the IGP leaves only. BGP pathlists
   and leaves are not modified. Hence traffic restoration occurs within
   the time frame of IGP convergence, and, for local link failure,
   assuming a backup path has been precomputed, within the timeframe of
   local detection (e.g. 50ms). Examples of solutions that pre-
   computing backup paths are IP FRR [16] remote LFA [17], Ti-LFA [15]
   and MRT [18] or eBGP path having a backup path [10].

   Let's apply the procedure mentioned in this subsection to the
   forwarding chain depicted in Figure 2. Suppose a remote link failure
   occurs and impacts the first ECMP IGP path to the remote BGP next-
   hop. Upon IGP convergence, the IGP pathlist used by the BGP next-hop
   is updated to reflect the new topology (one path instead of two). As
   soon as the IGP convergence is effective for the BGP next-hop entry,
   the new forwarding state is immediately available to all dependent
   BGP prefixes. The same behavior would occur if the failure was local
   such as an interface going down. As soon as the IGP convergence is
   complete for the BGP next-hop IGP route, all its BGP depending
   routes benefit from the new path. In fact, upon local failure, if
   LFA protection is enabled for the IGP route to the BGP next-hop and
   a backup path was pre-computed and installed in the pathlist, upon
   the local interface failure, the LFA backup path is immediately
   activated (e.g. sub-50msec) and thus protection benefits all the


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   depending BGP traffic through the hierarchical forwarding dependency
   between the routes.

6.2. BGP-PIC edge

   This section describes the adjustments to the forwarding chains as a
   result of edge node or edge link failure.



6.2.1. Adjusting forwarding Chain in egress node failure

   When an edge node fails, IGP on neighboring core nodes send route
   updates indicating that the edge node is no longer reachable. IGP
   running on the iBGP peers instructs FIB to remove the IP and label
   leaves corresponding to the failed edge node from FIB. So FIB
   manager performs the following steps:

   o  FIB manager deletes the IGP leaf corresponding to the failed edge
      node

   o  FIB manager backwalks to all dependent BGP pathlists and marks
      that path using the deleted IGP leaf as unresolved

   o  Note that there is no need to modify the possibly large number of
      BGP leaves because each path in the pathlist carries its path
      index and hence the correct outgoing label will be picked.
      Consider for example the forwarding chain depicted in Figure 2.
      If the 1st BGP path becomes unresolved, then the forwarding
      engine will only use the second path for forwarding. Yet the path
      index of that single resolved path will still be 1 and hence the
      label VPN-L12 will be pushed.

6.2.2. Adjusting Forwarding Chain on PE-CE link Failure

   Suppose the link between an edge router and its external peer fails.
   There are two scenarios (1) the edge node attached to the failed
   link performs next-hop self and (2) the edge node attached to the
   failure advertises the IP address of the failed link as the next-hop
   attribute to its iBGP peers.

   In the first case, the rest of iBGP peers will remain unaware of the
   link failure and will continue to forward traffic to the edge node
   until the edge node attached to the failed link withdraws the BGP
   prefixes. If the destination prefixes are multi-homed to another
   iBGP peer, say ePE2, then FIB manager on the edge router detecting
   the link failure applies the following steps:

   o  FIB manager backwalks to the BGP pathlists marks the path through
      the failed link to the external peer as unresolved

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   o  Hence traffic will be forwarded used the backup path towards ePE2

   o  For labeled traffic

       o The Outlabel-List attached to the BGP leaf already contains
          an entry corresponding to the backup path.

       o The label entry in OutLabel-List corresponding to the
          internal path to backup egress PE has swap action to the
          label advertised by backup egress PE

       o For an arriving label packet (e.g. VPN), the top label is
          swapped with the label advertised by backup egress PE and the
          packet is sent towards that backup egress PE

   o  For unlabeled traffic, packets are simply redirected towards
      backup egress PE.

   In the second case where the edge router uses the IP address of the
   failed link as the BGP next-hop, the edge router will still perform
   the previous steps. But, unlike the case of next-hop self, IGP on
   failed edge node informs the rest of the iBGP peers that IP address
   of the failed link is no longer reachable. Hence the FIB manager on
   iBGP peers will delete the IGP leaf corresponding to the IP prefix
   of the failed link. The behavior of the iBGP peers will be identical
   to the case of edge node failure outlined in Section 6.2.1.

   It is noteworthy to mention that because the edge link failure is
   local to the edge router, sub-50 msec convergence can be achieved as
   described in [10].

   Let's try to apply the case of next-hop self to the forwarding chain
   depicted in Figure 3. After failure of the link between ePE1 and CE,
   the forwarding engine will route traffic arriving from the core
   towards VPN-NH2 with path-index=1. A packet arriving from the core
   will contain the label VPN-L11 at top. The label VPN-L11 is swapped
   with the label VPN-L21 and the packet is forwarded towards ePE2.

6.3. Handling Failures for Flattened Forwarding Chains

   As explained in the in Section 5 if the number of hierarchy levels
   of a platform cannot support the native number of hierarchy levels
   of a recursive forwarding chain, the instantiated forwarding chain
   is constructed by flattening two or more levels. Hence a 3 levels
   chain in Figure 5 is flattened into the 2 levels chain in Figure 6.

   While reducing the benefits of BGP-PIC, flattening one hierarchy
   into a shallower hierarchy does not always result in a complete loss
   of the benefits of the BGP-PIC. To illustrate this fact suppose
   ASBR12 is no longer reachable in domain 1. If the platform supports

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   the full hierarchy depth, the forwarding chain is the one depicted
   in Figure 5 and hence the FIB manager needs to backwalk one level to
   the pathlist shared by "ePE1" and "ePE2" and adjust it. If the
   platform supports 2 levels of hierarchy, then a useable forwarding
   chain is the one depicted in Figure 6. In that case, if ASBR12 is no
   longer reachable, the FIB manager has to backwalk to the two
   flattened pathlists and updates both of them.

   The main observation is that the loss of convergence speed due to
   the loss of hierarchy depth depends on the structure of the
   forwarding chain itself. To illustrate this fact, let's take two
   extremes. Suppose the forwarding objects in level i+1 depend on the
   forwarding objects in level i. If every object on level i+1 depends
   on a separate object in level i, then flattening level i into level
   i+1 will not result in loss of convergence speed. Now let's take the
   other extreme. Suppose "n" objects in level i+1 depend on 1 object
   in level i. Now suppose FIB flattens level i into level i+1. If a
   topology change results in modifying the single object in level i,
   then FIB has to backwalk and modify "n" objects in the flattened
   level, thereby losing all the benefit of BGP-PIC. Experience shows
   that flattening forwarding chains usually results in moderate loss
   of BGP-PIC benefits. Further analysis is needed to corroborate and
   quantify this statement.

7. Properties

7.1. Coverage

   All the possible failures, except CE node failure, are covered,
   whether they impact a local or remote IGP path or a local or remote
   BGP next-hop as described in Section 6. This section provides
   details for each failure and now the hierarchical and shared FIB
   structure proposed in this document allows recovery that does not
   depend on number of BGP prefixes.

7.1.1. A remote failure on the path to a BGP next-hop

   Upon IGP convergence, the IGP leaf for the BGP next-hop is updated
   upon IGP convergence and all the BGP depending routes leverage the
   new IGP forwarding state immediately. Details of this behavior can
   be found in Section 6.1.

   This BGP resiliency property only depends on IGP convergence and is
   independent of the number of BGP prefixes impacted.

7.1.2. A local failure on the path to a BGP next-hop

   Upon LFA protection, the IGP leaf for the BGP next-hop is updated to
   use the precomputed LFA backup path and all the BGP depending routes


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   leverage this LFA protection. Details of this behavior can be found
   in Section 6.1.

   This BGP resiliency property only depends on LFA protection and is
   independent of the number of BGP prefixes impacted.

7.1.3. A remote iBGP next-hop fails

   Upon IGP convergence, the IGP leaf for the BGP next-hop is deleted
   and all the depending BGP Path-Lists are updated to either use the
   remaining ECMP BGP best-paths or if none remains available to
   activate precomputed backups. Details about this behavior can be
   found in Section 6.2.1.

   This BGP resiliency property only depends on IGP convergence and is
   independent of the number of BGP prefixes impacted.

7.1.4. A local eBGP next-hop fails

   Upon local link failure detection, the adjacency to the BGP next-hop
   is deleted and all the depending BGP pathlists are updated to either
   use the remaining ECMP BGP best-paths or if none remains available
   to activate precomputed backups. Details about this behavior can be
   found in Section 6.2.2.

   This BGP resiliency property only depends on local link failure
   detection and is independent of the number of BGP prefixes impacted.

7.2. Performance

   When the failure is local (a local IGP next-hop failure or a local
   eBGP next-hop failure), a pre-computed and pre-installed backup is
   activated by a local-protection mechanism that does not depend on
   the number of BGP destinations impacted by the failure. Sub-50msec
   is thus possible even if millions of BGP routes are impacted.

   When the failure is remote (a remote IGP failure not impacting the
   BGP next-hop or a remote BGP next-hop failure), an alternate path is
   activated upon IGP convergence. All the impacted BGP destinations
   benefit from a working alternate path as soon as the IGP convergence
   occurs for their impacted BGP next-hop even if millions of BGP
   routes are impacted.

   Appendix A puts the BGP PIC benefits in perspective by providing
   some results using actual numbers.




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7.3. Automated

   The BGP PIC solution does not require any operator involvement. The
   process is entirely automated as part of the FIB implementation.

   The salient points enabling this automation are:

   o  Extension of the BGP Best Path to compute more than one primary
      ([11]and [12]) or backup BGP next-hop ([6] and [13]).

   o  Sharing of BGP Path-list across BGP destinations with same
      primary and backup BGP next-hop

   o  Hierarchical indirection and dependency between BGP pathlist and
      IGP pathlist

7.4. Incremental Deployment

   As soon as one router supports BGP PIC solution, it benefits from
   all its benefits without any requirement for other routers to
   support BGP PIC.



8. Security Considerations

   The behavior described in this document is internal functionality
   to a router that result in significant improvement to convergence
   time as well as reduction in CPU and memory used by FIB while not
   showing change in basic routing and forwarding functionality. As
   such no additional security risk is introduced by using the
   mechanisms proposed in this document.

9. IANA Considerations

   No requirements for IANA

10. Conclusions

   This document proposes a hierarchical and shared forwarding chain
   structure that allows achieving BGP prefix independent
   convergence, and in the case of locally detected failures, sub-50
   msec convergence. A router can construct the forwarding chains in
   a completely transparent manner with zero operator intervention
   thereby supporting smooth and incremental deployment.






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11. References

11.1. Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

   [2]   Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol
         4 (BGP-4), RFC 4271, January 2006

   [3]   Bates, T., Chandra, R., Katz, D., and Rekhter Y.,
         "Multiprotocol Extensions for BGP", RFC 4760, January 2007

   [4]   Y. Rekhter and E. Rosen, " Carrying Label Information in BGP-
         4", RFC 3107, May 2001

   [5]   Andersson, L., Minei, I., and B. Thomas, "LDP Specification",
         RFC 5036, October 2007

11.2. Informative References

   [6]   Marques,P., Fernando, R., Chen, E, Mohapatra, P., Gredler, H.,
         "Advertisement of the best external route in BGP", draft-ietf-
         idr-best-external-05.txt, January 2012.

   [7]   Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
         Framework", RFC 5565, June 2009.

   [8]   Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
         Networks (VPNs)", RFC 4364, February 2006.

   [9]   De Clercq, J. , Ooms, D., Prevost, S., Le Faucheur, F.,
         "Connecting IPv6 Islands over IPv4 MPLS Using IPv6 Provider
         Edge Routers (6PE)", RFC 4798, February 2007

   [10]  O. Bonaventure, C. Filsfils, and P. Francois. "Achieving sub-
         50 milliseconds recovery upon bgp peering link failures, "
         IEEE/ACM Transactions on Networking, 15(5):1123-1135, 2007

   [11]  D. Walton, A. Retana, E. Chen, J. Scudder, "Advertisement of
         Multiple Paths in BGP", draft-ietf-idr-add-paths-12.txt,
         November 2015

   [12]  R. Raszuk, R. Fernando, K. Patel, D. McPherson, K. Kumaki,
         "Distribution of diverse BGP paths", RFC 6774, November 2012

   [13]  P. Mohapatra, R. Fernando, C. Filsfils, and R. Raszuk, "Fast
         Connectivity Restoration Using BGP Add-path", draft-pmohapat-
         idr-fast-conn-restore-03, Jan 2013


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   [14]  C. Filsfils, S. Previdi, A. Bashandy, B. Decraene, S.
         Litkowski, M. Horneffer, R. Shakir, J. Tansura, E. Crabbe
         "Segment Routing with MPLS data plane", draft-ietf-spring-
         segment-routing-mpls-02 (work in progress), October 2015

   [15]  C. Filsfils, S. Previdi, A. Bashandy, B. Decraene, " Topology
         Independent Fast Reroute using Segment Routing", draft-
         francois-spring-segment-routing-ti-lfa-02 (work in progress),
         August 2015

   [16]  M. Shand and S. Bryant, "IP Fast Reroute Framework", RFC 5714,
         January 2010

   [17]  S. Bryant, C. Filsfils, S. Previdi, M. Shand, N So, " Remote
         Loop-Free Alternate (LFA) Fast Reroute (FRR)", RFC 7490 April
         2015

   [18]  A. Atlas, C. Bowers, G. Enyedi, " An Architecture for IP/LDP
         Fast-Reroute Using Maximally Redundant Trees", draft-ietf-
         rtgwg-mrt-frr-architecture-10 (work in progress), February
         2016

12. Acknowledgments

   Special thanks to Neeraj Malhotra, Yuri Tsier for the valuable
   help

   Special thanks to Bruno Decraene for the valuable comments

   This document was prepared using 2-Word-v2.0.template.dot.



Authors' Addresses

   Ahmed Bashandy
   Cisco Systems
   170 West Tasman Dr, San Jose, CA 95134, USA
   Email: bashandy@cisco.com

   Clarence Filsfils
   Cisco Systems
   Brussels, Belgium
   Email: cfilsfil@cisco.com

   Prodosh Mohapatra
   Sproute Networks
   Email: mpradosh@yahoo.com



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Appendix A.                 Perspective

   The following table puts the BGP PIC benefits in perspective
   assuming

   o  1M impacted BGP prefixes

   o  IGP convergence ~ 500 msec

   o  local protection ~ 50msec

   o  FIB Update per BGP destination ~ 100usec conservative,

                                     ~ 10usec optimistic

   o  BGP Convergence per BGP destination ~ 200usec conservative,

                                          ~ 100usec optimistic



                                 Without PIC                With PIC

   Local IGP Failure             10 to 100sec                50msec

   Local BGP Failure            100 to 200sec                50msec

   Remote IGP Failure            10 to 100sec               500msec

   Local BGP Failure            100 to 200sec               500msec

   Upon local IGP next-hop failure or remote IGP next-hop failure, the
   existing primary BGP next-hop is intact and usable hence the
   resiliency only depends on the ability of the FIB mechanism to
   reflect the new path to the BGP next-hop to the depending BGP
   destinations. Without BGP PIC, a conservative back-of-the-envelope
   estimation for this FIB update is 100usec per BGP destination. An
   optimistic estimation is 10usec per entry.

   Upon local BGP next-hop failure or remote BGP next-hop failure,
   without the BGP PIC mechanism, a new BGP Best-Path needs to be
   recomputed and new updates need to be sent to peers. This depends on
   BGP processing time that will be shared between best-path
   computation, RIB update and peer update. A conservative back-of-the-
   envelope estimation for this is 200usec per BGP destination. An
   optimistic estimation is 100usec per entry.





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