Routing Area Working Group                                 A. Atlas, Ed.
Internet-Draft                                                 R. Kebler
Intended status: Standards Track                      M. Konstantynowicz
Expires: July 29, 2012                                  Juniper Networks
                                                               G. Enyedi
                                                              A. Csaszar
                                                                R. White
                                                           Cisco Systems
                                                                M. Shand
                                                        January 26, 2012

An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees


   As IP and LDP Fast-Reroute are increasingly deployed, the coverage
   limitations of Loop-Free Alternates are seen as a problem that
   requires a straightforward and consistent solution for IP and LDP,
   for unicast and multicast.  This draft describes an architecture
   based on redundant backup trees where a single failure can cut a
   point-of-local-repair from the destination only on one of the pair of
   redundant trees.

   One innovative algorithm to compute such topologies is maximally
   disjoint backup trees.  Each router can compute its next-hops for
   each pair of maximally disjoint trees rooted at each node in the IGP
   area with computational complexity similar to that required by

   The additional state, address and computation requirements are
   believed to be significantly less than the Not-Via architecture

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 29, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Goals for Extending IP Fast-Reroute coverage beyond LFA  .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Maximally Redundant Trees (MRT)  . . . . . . . . . . . . . . .  6
   4.  Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . .  8
     4.1.  Multi-homed Prefixes . . . . . . . . . . . . . . . . . . .  9
     4.2.  Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . 10
       4.2.1.  LDP Unicast Forwarding - Avoid Tunneling . . . . . . . 11  Protocol Extensions and Considerations: LDP  . . . 12
       4.2.2.  IP Unicast Traffic . . . . . . . . . . . . . . . . . . 12  Protocol Extensions and Considerations: OSPF
                   and ISIS . . . . . . . . . . . . . . . . . . . . . 13
       4.2.3.  Inter-Area and ABR Forwarding Behavior . . . . . . . . 13
       4.2.4.  Issues with Area Abstraction . . . . . . . . . . . . . 15
       4.2.5.  Partial Deployment and Islands of Compatible MRT
               FRR routers  . . . . . . . . . . . . . . . . . . . . . 16
       4.2.6.  Network Convergence and Preparing for the Next
               Failure  . . . . . . . . . . . . . . . . . . . . . . . 17  Micro-forwarding loop prevention and MRTs  . . . . 17  MRT Recalculation  . . . . . . . . . . . . . . . . 17
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   There is still work required to completely provide IP and LDP Fast-
   Reroute[RFC5714] for unicast and multicast traffic.  This draft
   proposes an architecture to provide 100% coverage.

   Loop-free alternates (LFAs)[RFC5286] provide a useful mechanism for
   link and node protection but getting complete coverage is quite hard.
   [LFARevisited] defines sufficient conditions to determine if a
   network provides link-protecting LFAs and also proves that augmenting
   a network to provide better coverage is NP-hard.
   [I-D.ietf-rtgwg-lfa-applicability] discusses the applicability of LFA
   to different topologies with a focus on common PoP architectures.

   While Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as
   an architecture, in practice, it has proved too complicated and
   stateful to spark substantial interest in implementation or
   deployment.  Academic implementations [LightweightNotVia] exist and
   have found the address management complexity high (but no
   standardization has been done to reduce this).

   A different approach is needed and that is what is described here.
   It is based on the idea of using disjoint backup topologies as
   realized by Maximally Redundant Trees (described in
   [LightweightNotVia]); the general architecture could also apply to
   future improved redundant tree algorithms.

1.1.  Goals for Extending IP Fast-Reroute coverage beyond LFA

   Any scheme proposed for extending IPFRR network topology coverage
   beyond LFA, apart from attaining basic IPFRR properties, should also
   aim to achieve the following usability goals:

   o  ensure maximum physically feasible link and node disjointness
      regardless of topology,

   o  automatically compute backup next-hops based on the topology
      information distributed by link-state IGP,

   o  do not require any signaling in the case of failure and use pre-
      programmed backup next-hops for forwarding,

   o  introduce minimal amount of additional addressing and state on

   o  enable gradual introduction of the new scheme and backward

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   o  and do not impose requirements for external computation.

2.  Terminology

   2-connected:   A graph that has no cut-vertices.  This is a graph
      that requires two nodes to be removed before the network is

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

   2-edge-connected:   A network graph where at least two links must be
      removed to partition the network.

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

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

   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.

   cut-vertex:   A vertex whose removal partitions the network.

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

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

   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.
      Any RT is an MRT but many MRTs are not RTs.

   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.

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   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.
      These can be computed in 2-connected graphs.

3.  Maximally Redundant Trees (MRT)

   In the last few years, there's been substantial research on how to
   compute and use redundant trees.  Redundant trees are directed
   spanning trees that provide disjoint paths towards their common root.
   These redundant trees only exist and provide link protection if the
   network is 2-edge-connected and node protection if the network is
   2-connected.  Such connectiveness may not be the case in real
   networks, either due to architecture or due to a previous failure.
   The work on maximally redundant trees has added two useful pieces
   that make them ready for use in a real network.

   o  Computable regardless of network topology: The maximally redundant
      trees are computed so that only the cut-edges or cut-vertices are
      shared between the multiple trees.

   o  Computationally practical algorithm is based on a common network
      topology database.  Algorithm variants can compute in O( e) or O(e
      + n log n), as given in [I-D.enyedi-rtgwg-mrt-frr-algorithm].

   There is, of course, significantly more in the literature related to
   redundant trees and even fast-reroute, but the formulation of the
   Maximally Redundant Trees (MRT) algorithm makes it very well suited
   to use in routers.

   A known disadvantage of MRT, and redundant trees in general, is that
   the trees do not necessarily provide shortest detour paths.  The use
   of the shortest-path-first algorithm in tree-building and including
   all links in the network as possibilities for one path or another
   should improve this.  Modeling is underway to investigate and compare
   the MRT alternates to the optimal
   [I-D.enyedi-rtgwg-mrt-frr-algorithm].  Providing shortest detour
   paths would require failure-specific detour paths to the
   destinations, but the state-reduction advantage of MRT lies in the
   detour being established per destination (root) instead of per
   destination AND per failure.

   The specific algorithm to compute MRTs as well as the logic behind
   that algorithm and alternative computational approaches are given in
   detail in [I-D.enyedi-rtgwg-mrt-frr-algorithm].  Those interested are
   highly recommended to read that document.  This document describes
   how the MRTs can be used and not how to compute them.

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   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.  The two paths along the two MRTs to a
   given destination-root of a 2-connected graph are node-disjoint,
   while in any non-2-connected graph, only the cut-vertices and cut-
   edges can be contained by both of the paths.

   For example, in Figure 1, there is a network graph that is
   2-connected in (a) and associated MRTs in (b) and (c).  One can
   consider the paths from B to R; on the Blue MRT, the paths are
   B->F->D->E->R or B->F->C->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 1: A 2-connected Network

   By contrast, in Figure 2, the network in (a) is not 2-conneted.  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 1 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.

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                       |     |    |     |----[I]
                       |     |    |     |     |
                      [R]---[C]  [F]---[G]    |
                       |     |    |     |     |
                       |     |    |     |----[J]

                        a non-2-connected graph

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

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

                    Figure 2: A non-2-connected network

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

   In normal IGP routing, each router has its shortest-path-tree to all
   destinations.  From the perspective of a particular destination, D,
   this looks like a reverse SPT (rSPT).  To use maximally redundant
   trees, in addition, each destination D has two MRTs associated with
   it; by convention these will be called the blue and red MRTs.

   MRTs are practical to maintain redundancy even after a single link or
   node failure.  If a pair of MRTs is computed rooted at each
   destination, all the destinations remain reachable along one of the
   MRTs in the case of a single link or node failure.

   When there is a link or node failure affecting the rSPT, each node
   will still have at least one path via one of the MRTs to reach the
   destination D. For example, in Figure 2, C would normally forward
   traffic to R across the C<->R link.  If that C<->R link fails, then C
   could use either the Blue MRT path C->D->E->R or the Red MRT path

   As is always the case with fast-reroute technologies, forwarding does
   not change until a local failure is detected.  Packets are forwarded

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   along the shortest path.  The appropriate alternate to use is pre-
   computed.  [I-D.enyedi-rtgwg-mrt-frr-algorithm] describes exactly how
   to determine whether the Blue MRT next-hops or the Red MRT next-hops
   should be the MRT alternate next-hops for a particular primary next-
   hop N to a particular destination D.

   MRT alternates are always available to use, unless the network has
   been partitioned.  It is a local decision whether to use an MRT
   alternate, a Loop-Free Alternate or some other type of alternate.
   When a network needs to use a micro-loop prevention mechanism
   [RFC5715] such as Ordered FIB[I-D.ietf-rtgwg-ordered-fib] or Farside
   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
   impacting traffic badly.

   As described in [RFC5286], when a worse failure than is anticipated
   happens, using LFAs that are not downstream neighbors can cause
   micro-looping.  An example is given of link-protecting alternates
   causing a loop on node failure.  Even if a worse failure than
   anticipated happened, the use of MRT alternates will not cause
   looping.  Therefore, while node-protecting LFAs may be prefered,
   there are advantages to using MRT alternates when such a node-
   protecting LFA is not a downstream path.

4.1.  Multi-homed Prefixes

   One advantage of LFAs that is necessary to preserve is the ability 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 also be
   done for backups via MRT.

   This generalizes to any multi-homed prefix.  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.

   For each prefix, the two lowest total cost ABRs are selected and a
   proxy-node is created connected to those two ABRs.  If there exist
   multiple multi-homed prefixes that share the same two best
   connectivity, then a single proxy-node can be used to represent the

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   set.  An example of this is shown in Figure 3.

                    2    2                     2     2
                  A----B----C                A----B----C
                2 |         | 2            2 |         | 2
                  |         |                |         |
                [ABR1]    [ABR2]           [ABR1]    [ABR2]
                  |         |                |         |
                 p,10      p,15           10 |---[P]---| 15

                (a) Initial topology         (b)with proxy-node

                A<---B<---C                 A--->B--->C
                |         ^                 ^         |
                V         |                 |         V
              [ABR1]    [ABR2]            [ABR1]    [ABR2]
                |                                     |
                |-->[P]                         [P]<--|

                (c) Blue MRT                (d) Red MRT

              Figure 3: Prefixes Advertised by Multiple ABRs

   The proxy-nodes and associated links are added to the network
   topology after all real links have been assigned to a direction and
   before the actual MRTs are computed.  Proxy-nodes cannot be transited
   when computing the MRTs.  In addition to computing the pair of MRTs
   associated with each router destination D in the area, a pair of MRTs
   can be computed for each such proxy-node to fully protect against ABR

   Each ABR or attaching router must remove the MRT marking[see
   Section 4.2] and then forward the traffic outside of the area (or
   island of MRT-fast-reroute-supporting routers).

   When directing traffic along an MRT towards a multi-homed prefix, if
   a topology-identifier label[see Section 4.2.1] is not used, then the
   proxy-node must be named and either additional LDP labels or IP
   addresses associated with it.

4.2.  Unicast Forwarding with MRT Fast-Reroute

   With LFA, there is no need to tunnel unicast traffic, whether IP or
   LDP.  The traffic is simply sent to an alternate.  The behavior with
   MRT Fast-Reroute is different depending upon whether IP or LDP
   unicast traffic is considered.

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   Logically, one could use the same IP address or LDP FEC and then also
   use 2 bits to express the topology to use.  The topology options are
   (00) IGP/SPT, (01) blue MRT, (10) red MRT.  Unfortunately, there just
   aren't 2 spare bits available in the IPv4 or IPv6 header.  This has
   different consequences for IP and LDP because LDP can just add a
   topology label on top or take 2 spare bits from the label space.

   Once the MRTs are computed, the two sets of MRTs are seen by the
   forwarding plane as essentially two additional topologies.  The same
   considerations apply for forwarding along the MRTs as for handling
   multiple topologies.

4.2.1.  LDP Unicast Forwarding - Avoid Tunneling

   For LDP, it is very desirable to avoid tunneling because, for at
   least node protection, tunneling requires knowledge of remote LDP
   label mappings and thus requires targeted LDP sessions and the
   associated management complexity.  There are two different mechanisms
   that can be used.

   1.  Option A - Encode Topology in Labels: In addition to sending a
       single label for a FEC, a router would provide two additional
       labels with their associated MRT colors.  This is simple, but
       reduces the label space for other uses.  It also increases the
       memory to store the labels and the communication required by LDP.

   2.  Option B - Create Topology-Identification Labels: Use the label-
       stacking ability of MPLS and specify only two additional labels -
       one for each associated MRT color - by a new FEC type.  When
       sending a packet onto an MTR, first swap the LDP label and then
       push the topology-identification label for that MTR color.  When
       receiving a packet with a topology-identification label, pop it
       and use it to guide the next-hop selection in combination with
       the next label in the stack; then swap the remaining label, if
       appropriate, and push the topology-identification label for the
       next-hop.  This 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 can use the same mechanisms as are needed for context-aware
       label spaces.

   Note that with LDP unicast forwarding, regardless of whether
   topology-identification label or encoding topology in label is used,
   no additional loopbacks per router are required as are required in
   the IP unicast forwarding case.  This is because LDP labels are used
   on a hop-by-hop basis to identify MRT-blue and MRT-red forwarding

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   For greatest hardware compatibility, routers should support Option B
   of encoding the topology in the labels.  Protocol Extensions and Considerations: LDP

   This captures an initial understanding of what may need to be

   1.  Specify Topology in Label: When sending a Label Mapping, have the
       ability to send a Label TLV and multiple Topology-Label TLVs.
       The Topology-Label TLV would specify MRT and the associated MRT

   2.  Topology-Identification Labels: Define a new FEC type that
       describes the topology for MRT and the associated MRT color.

4.2.2.  IP Unicast Traffic

   For IP, there is no currently practical alternative except tunneling.
   The tunnel egress could be the original destination in the area, the
   next-next-hop, etc..  If the tunnel egress is the original
   destination router, then the traffic remains on the redundant tree
   with sub-optimal routing.  If the tunnel egress is the next-next-hop,
   then protection of multi-homed prefixes and node-failure for ABRs is
   not available.  Selection of the tunnel egress is a router-local

   There are three options available for marking IP packets with which
   MRT it should be forwarded in.

   1.  Tunnel IP packets via an LDP LSP.  This has the advantage that
       more installed routers can do line-rate encapsulation and
       decapsulation.  Also, no additional IP addresses would need to be
       allocated or signaled.

       A.  Option A - LDP Destination-Topology Label: Use a label that
           indicates both destination and MRT.  This method allows easy
           tunneling to the next-next-hop as well as to the IGP-area
           destination.  For multi-homed prefixes, this requires that
           additional labels be advertised for each proxy-node.

       B.  Option B - LDP Topology Label: Use a Topology-Identifier
           label on top of the IP packet.  This is very simple and
           doesn't require additional labels for proxy-nodes.  If
           tunneling to a next-next-hop is desired, then a two-deep
           label stack can be used with [ Topology-ID label, Next-Next-
           Hop Label ].

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   2.  Tunnel IP packets in IP.  Each router supporting this option
       would announce 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.  They
       allow the transit nodes to identify the traffic as being
       forwarded along either MRT-blue or MRT-red tree topology to reach
       the tunnel destination.  Announcements of these two additional
       loopback addresses per router with their MRT color requires IGP

   For proxy-nodes associated with one or more multi-homed prefixes, the
   problem is harder because there is no router associated with the
   proxy-node, so its loopbacks can't be known or used.  In this case,
   each router attached to the proxy-node could announce two common IP
   addresses with their associated MRT colors.  This would require
   configuration as well as the previously mentioned IGP extensions.
   Similarly, in the LDP case, two additional FEC bindings could be
   announced.  Protocol Extensions and Considerations: OSPF and ISIS

   This captures an initial understanding of what may need to be

   o  Capabilities: Does a router support MRT?  Does the router do MRT
      tunneling with LDP or IP or GRE or...?

   o  Topology Association: A router needs to advertise a loopback and
      associate it with an MRT whether blue or red.  Additional
      flexibility for future uses would be good.

   o  Proxy-nodes for Multi-homed Prefixes: We need a way to advertise
      common addresses with MRT for multi-homed prefixes' proxy-nodes.
      Currently, those proxy-nodes aren't named or considered.

   As with LFA, it is expected that OSPF Virtual Links will not be

4.2.3.  Inter-Area and ABR Forwarding Behavior

   In regular forwarding, packets destined outside the area arrive at
   the ABR and the ABR forwards them into the other area because the
   next-hops from the area with the best route (according to tie-
   breaking rules) are used by the ABR.  The question is then what to do
   with packets marked with an MRT that are received by the ABR.

   The only option that doesn't require forwarding based upon incoming
   interface is to forward an MRT marked packet in the area with the

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   best route along its associated MRT.  If the packet came from that
   area, this correctly avoids the failure.  If the packet came from a
   different area, at least this gets the packet to the destination even
   though it is along an MRT rather than the shortest-path.

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

         (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
                |                                |

                  (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

   To avoid using an out-of-area MRT, special action can be taken by the
   penultimate router along the in-local-area MRT immediately before the
   ABR is reached.  The penultimate router can determine that the ABR

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   will forward the packet out of area and, in that case, the
   penultimate router can remove the MRT marking but still forward the
   packet along the MRT next-hop to reach the ABR.  For instance, in
   Figure 4, if node H fails, node E has to put traffic towards prefix p
   onto the red MRT.  But since node D knows that ABR1 will use a best
   from another area, it is safe for D to remove the MRT marking and
   just send the packet to ABR1 still on the red MRT but unmarked.  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 this is a realistic
   deployment scenario, OSPF extensions could be considered.

4.2.4.  Issues with Area Abstraction

   MRT fast-reroute provides complete coverage in a area that is
   2-connected.  Where a failure would partition the network, of course,
   no alternate can protect against that failure.  Similarly, there are
   ways of connecting multi-homed prefixes that make it impractical to
   protect them without excessive complexity.

         |----[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.  The only real way to get it from A to ASBR Y is to explicitly
   tunnel it to ASBR Y.

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   Tunnelling to the backup ASBR is for future consideration.  The
   previously proposed PHP approach needs to have an exception if BGP
   policies (e.g.  BGP local preference) determines which ASBR to use.
   Consider the case in Figure 6.  If the link between A and ASBR X (the
   preferred border router) fails, A can put the packets to p onto an
   MRT alternate, even tunnel it towards ASBR Y. Node B, however, must
   not remove the MRT marking in this case, as nodes in Area 0,
   including ASBR Y itself would not know that their preferred ASBR is

                      Area 20                    BB Area 0
          p ---[ASBR X]-X-[A]---[B]---[ABR 1]---[D]---[ASBR Y]--- p

                      BGP prefers ASBR X for prefix p

          Figure 6: Failure of path towards ASBR preferred by BGP

   The fine details of how to solve multi-area external prefix cases, or
   identifying certain cases as too unlikely and too complex to protect
   is for further consideration.

4.2.5.  Partial Deployment and Islands of Compatible MRT FRR routers

   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.

   First, a computing router S must determine its local island of
   compatible MRT fast-reroute routers.  A router that has common
   forwarding mechanisms and common algorithm and is connected to either
   to S or to another router already determined to be in S's local
   island can be added to S's local island.

   Destinations inside the local island can obviously use MRT
   alternates.  Destinations outside the local island can be treated
   like a multi-homed prefix with caveats to avoid looping.  For LDP
   labels including both destination and topology, the routers at the
   borders of the local island need to originate labels for the original
   FEC and the associated MRT-specific labels.  Packets sent to an LDP
   label marked as blue or red MRT to a destination outside the local
   island will have the last router in the local island swap the label
   to one for the destination and forward the packet along the outgoing

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   interface on the MRT towards a router outside the local island that
   was represented by the proxy-node.

   For IP in IP encapsulations, remote destinations may not be
   advertising additional IP loopback addresses for the MRTs.  In that
   case, a router attached to a proxy-node, which represents
   destinations outside the local island, must advertise IP addresses
   associated with that proxy-node.  Packets sent to an address
   associated with a proxy-node will have their outer IP header removed
   by the router attached to the proxy-node and be forwarded by the
   router along the outgoing interface on the MRT towards a router
   outside the local island that was represented by the proxy-node.

4.2.6.  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
   the new shortest paths.  Two things need attention: micro-loop
   prevention and MRT re-calculation.  Micro-forwarding loop prevention and MRTs

   As is well known[RFC5715], micro-loops can occur during IGP
   convergence; such loops can be local to the failure or remote from
   the failure.  Managing micro-loops is an orthogonal issue to having
   alternates for local repair, such as MRT fast-reroute provides.

   There are two possible micro-loop prevention mechanism discussed in
   [RFC5715].  The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib].
   The second is Farside Tunneling which requires tunnels or an
   alternate topology to reach routers on the farside of the failure.

   Since MRTs provide an alternate topology through which traffic can be
   sent and which can be manipulated separately from the SPT, it is
   possible that MRTs could be used to support Farside Tunneling.
   Details of how to do so are outside of this document.  MRT Recalculation

   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:

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   1.  Receive failure notification

   2.  Recompute SPT

   3.  Install new SPT

   4.  Recompute MRTs

   5.  Wait configured period for all routers to be using their SPTs and
       traffic to drain from the MRTs.

   6.  Install new MRTs.

   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 as quickly as possible.

   It is for further study whether MRT re-calculation is possible in an
   incremental fashion, such that the sections of the MRT in use after a
   failure are not changed.

5.  Acknowledgements

   The authors would like to thank Hannes Gredler, Jeff Tantsura, Ted
   Qian, Kishore Tiruveedhula, Santosh Esale, Nitin Bahadur, Harish
   Sitaraman and Raveendra Torvi for their suggestions and review.

6.  IANA Considerations

   This doument includes no request to IANA.

7.  Security Considerations

   This architecture is not currently believed to introduce new security

8.  References

8.1.  Normative References

              Atlas, A., Envedi, G., and A. Csaszar, "Algorithms for
              computing Maximally Redundant Trees for IP/LDP Fast-
              Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-00 (work in

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              progress), October 2011.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [RFC5384]  Boers, A., Wijnands, I., and E. Rosen, "The Protocol
              Independent Multicast (PIM) Join Attribute Format",
              RFC 5384, November 2008.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, January 2010.

8.2.  Informative References

              Bryant, S., Previdi, S., and M. Shand, "IP Fast Reroute
              Using Not-via Addresses",
              draft-ietf-rtgwg-ipfrr-notvia-addresses-08 (work in
              progress), December 2011.

              Filsfils, C. and P. Francois, "LFA applicability in SP
              networks", draft-ietf-rtgwg-lfa-applicability-06 (work in
              progress), January 2012.

              Shand, M., Bryant, S., Previdi, S., and C. Filsfils,
              "Loop-free convergence using oFIB",
              draft-ietf-rtgwg-ordered-fib-05 (work in progress),
              April 2011.

              Retvari, G., Tapolcai, J., Enyedi, G., and A. Csaszar, "IP
              Fast ReRoute: Loop Free Alternates Revisited", Proceedings
              of IEEE INFOCOM , 2011, <

              Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
              Fast ReRoute: Lightweight Not-Via without Additional
              Addresses", Proceedings of IEEE INFOCOM , 2009,

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, January 2010.

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Authors' Addresses

   Alia Atlas (editor)
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886


   Robert Kebler
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886


   Maciek Konstantynowicz
   Juniper Networks


   Gabor Sandor Enyedi
   Konyves Kalman krt 11.
   Budapest  1097


   Andras Csaszar
   Konyves Kalman krt 11
   Budapest  1097


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   Russ White
   Cisco Systems


   Mike Shand


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