Routing Area Working Group                                 A. Atlas, Ed.
Internet-Draft                                                        BT
Intended status: Standards Track                           A. Zinin, Ed.
Expires: September 28, 2008                                      Alcatel
                                                            Mar 27, 2008

     Basic Specification for IP Fast-Reroute: Loop-free Alternates

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This document describes the use of loop-free alternates to provide
   local protection for unicast traffic in pure IP and MPLS/LDP networks
   in the event of a single failure, whether link, node or shared risk
   link group (SRLG).  The goal of this technology is to reduce the
   packet loss that happens while routers converge after a topology
   change due to a failure.  Rapid failure repair is achieved through
   use of precalculated backup next-hops that are loop-free and safe to

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   use until the distributed network convergence process completes.
   This simple approach does not require any support from other routers.
   The extent to which this goal can be met by this specification is
   dependent on the topology of the network.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Failure Scenarios  . . . . . . . . . . . . . . . . . . . .  5
   2.  Applicability of Described Mechanisms  . . . . . . . . . . . .  8
   3.  Alternate Next-Hop Calculation . . . . . . . . . . . . . . . .  9
     3.1.  Basic Loop-free Condition  . . . . . . . . . . . . . . . . 10
     3.2.  Node-Protecting Alternate Next-Hops  . . . . . . . . . . . 10
     3.3.  Broadcast and NBMA Links . . . . . . . . . . . . . . . . . 11
     3.4.  ECMP and Alternates  . . . . . . . . . . . . . . . . . . . 12
     3.5.  Interactions with ISIS Overload, RFC 3137 and Costed
           Out Links  . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.5.1.  Interactions with ISIS Link Attributes . . . . . . . . 14
     3.6.  Selection Procedure  . . . . . . . . . . . . . . . . . . . 14
     3.7.  LFA types and Trade-offs . . . . . . . . . . . . . . . . . 18
     3.8.  A Simplification: Per-Next-Hop LFAs  . . . . . . . . . . . 19
   4.  Using an Alternate . . . . . . . . . . . . . . . . . . . . . . 20
     4.1.  Terminating Use of Alternate . . . . . . . . . . . . . . . 20
   5.  Requirements on LDP Mode . . . . . . . . . . . . . . . . . . . 22
   6.  Routing Aspects  . . . . . . . . . . . . . . . . . . . . . . . 22
     6.1.  Multi-Homed Prefixes . . . . . . . . . . . . . . . . . . . 22
     6.2.  ISIS . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     6.3.  OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
       6.3.1.  OSPF External Routing  . . . . . . . . . . . . . . . . 24
       6.3.2.  OSPF Multi-Topology  . . . . . . . . . . . . . . . . . 25
     6.4.  BGP Next-Hop Synchronization . . . . . . . . . . . . . . . 25
     6.5.  Multicast Considerations . . . . . . . . . . . . . . . . . 25
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     10.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Appendix A.  OSPF Example Where LFA Based on Local Area
                Topology is Insufficient  . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
   Intellectual Property and Copyright Statements . . . . . . . . . . 31

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1.  Introduction

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.  [RFC2119]

   Applications for interactive multimedia services such as VoIP and
   pseudo-wires can be very sensitive to traffic loss, such as occurs
   when a link or router in the network fails.  A router's convergence
   time is generally on the order of hundreds of milliseconds; the
   application traffic may be sensitive to losses greater than tens of

   As discussed in [I-D.ietf-rtgwg-ipfrr-framework], minimizing traffic
   loss requires a mechanism for the router adjacent to a failure to
   rapidly invoke a repair path, which is minimally affected by any
   subsequent re-convergence.  This specification describes such a
   mechanism which allows a router whose local link has failed to
   forward traffic to a pre-computed alternate until the router installs
   the new primary next-hops based upon the changed network topology.
   The terminology used in this specification is given in
   [I-D.ietf-rtgwg-ipfrr-framework].  The described mechanism assumes
   that routing in the network is performed using a link-state routing
   protocol-- OSPF [RFC2328] [RFC2740] [I-D.ietf-ospf-ospfv3-update] or
   ISIS [RFC1195] [RFC2966] (for IPv4 or IPv6).  The mechanism also
   assumes that both the primary path and the alternate path are in the
   same routing area.

   When a local link fails, a router currently must signal the event to
   its neighbors via the IGP, recompute new primary next-hops for all
   affected prefixes, and only then install those new primary next-hops
   into the forwarding plane.  Until the new primary next-hops are
   installed, traffic directed towards the affected prefixes is
   discarded.  This process can take hundreds of milliseconds.

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                        /------|  S  |--\
                       /       +-----+   \
                      / 5               8 \
                     /                     \
                   +-----+                +-----+
                   |  E  |                | N_1 |
                   +-----+                +-----+
                      \                     /
                  \    \  4              3 /  /
                   \|   \                 / |/
                   -+    \    +-----+    /  +-
                          \---|  D  |---/

                         Figure 1: Basic Topology

   The goal of IP Fast-Reroute is to reduce failure reaction time to 10s
   of milliseconds by using a pre-computed alternate next-hop, in the
   event that the currently selected primary next-hop fails, so that the
   alternate can be rapidly used when the failure is detected.  A
   network with this feature experiences less traffic loss and less
   micro-looping of packets than a network without IPFRR.  There are
   cases where traffic loss is still a possibility since IPFRR coverage
   varies but in the worst possible situation a network with IPFRR is
   equivalent with respect to traffic convergence to a network without

   To clarify the behavior of IP Fast-Reroute, consider the simple
   topology in Figure 1.  When router S computes its shortest path to
   router D, router S determines to use the link to router E as its
   primary next-hop.  Without IP Fast-Reroute, that link is the only
   next-hop that router S computes to reach D. With IP Fast-Reroute, S
   also looks for an alternate next-hop to use.  In this example, S
   would determine that it could send traffic destined to D by using the
   link to router N_1 and therefore S would install the link to N_1 as
   its alternate next-hop.  At some later time, the link between router
   S and router E could fail.  When that link fails, S and E will be the
   first to detect it.  On detecting the failure, S will stop sending
   traffic destined for D towards E via the failed link, and instead
   send the traffic to S's pre-computed alternate next-hop, which is the
   link to N_1, until a new SPF is run and its results are installed.
   As with the primary next-hop, an alternate next-hop is computed for
   each destination.  The process of computing an alternate next-hop
   does not alter the primary next-hop computed via a standard SPF.

   If in the example of Figure 1, the link cost from N_1 to D increased

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   to 30 from 3, then N_1 would not be a loop-free alternate, because
   the cost of the path from N_1 to D via S would be 17 while the cost
   from N_1 directly to D would be 30.  In real networks, we may often
   face this situation.  The existence of a suitable loop-free alternate
   next-hop is dependent on the topology and the nature of the failure
   the alternate is calculated for.

   This specification uses the terminology introduced in
   [I-D.ietf-rtgwg-ipfrr-framework].  In particular, it uses
   Distance_opt(X,Y), abbreviated to D_opt(X,Y), to indicate the
   shortest distance from X to Y. S is used to indicate the calculating
   router.  N_i is a neighbor of S; N is used as an abbreviation when
   only one neighbor is being discussed.  D is the destination under

   A neighbor N can provide a loop-free alternate (LFA) if and only if

        Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)

                     Inequality 1: Loop-Free Criterion

   A sub-set of loop-free alternate are downstream paths which must meet
   a more restrictive condition that is applicable to more complex
   failure scenarios:

                 Distance_opt(N, D) < Distance_opt(S, D)

                  Inequality 2: Downstream Path Criterion

1.1.  Failure Scenarios

   The alternate next-hop can protect against a single link failure, a
   single node failure, failure of one or more links within a shared
   risk link group failures, or a combination of these.  Whenever a
   failure occurs that is more extensive than what the alternate was
   intended to protect, there is the possibility of temporarily looping
   traffic (note again, that such a loop would only last until the next
   complete SPF calculation).  The example where a node fails when the
   alternate provided only link protection is illustrated below.  If
   unexpected simultaneous failures occur, then micro-looping may occur
   since the alternates are not pre-computed to avoid the set of failed

   If only link protection is provided and the node fails, it is
   possible for traffic using the alternates to experience micro-
   looping.  This issue is illustrated in Figure 2.  If Link(S->E)
   fails, then the link-protecting alternate via N will work correctly.
   However, if router E fails, then both S and N will detect a failure

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   and switch to their alternates.  In this example, that would cause S
   to redirect the traffic to N and N to redirect the traffic to S and
   thus causing a forwarding loop.  Such a scenario can arise because
   the key assumption, that all other routers in the network are
   forwarding based upon the shortest path, is violated because of a
   second simultaneous correlated failure - another link connected to
   the same primary neighbor.  If there are not other protection
   mechanisms to handle node failure, a node failure is still a concern
   when only using link protecting LFAs.

                    +-----+       +-----+
                    |  S  |-------|  N  |
                    +-+---+   5   +-----+
                      |              |
                      | 5          4 |  |
                   |  |              | \|/
                  \|/ |              |
                      |    +-----+   |
                      +----|  E  |---+
                              | 10
                           |  D  |

     Figure 2: Link-Protecting Alternates Causing Loop on Node Failure

   Micro-looping of traffic via the alternates caused when a more
   extensive failure than planned for occurs can be prevented via
   selection of only downstream paths as alternates.  A micro-loop due
   to the use of alternates can be avoided by using downstream paths
   because each succeeding router in the path to the destination must be
   closer to the destination than its predecessor (according to the
   topology prior to the failures).  Although use of downstream paths
   ensures that the micro-looping via alternates does not occur, such a
   restriction can severely limit the coverage of alternates.  In
   Figure 2, S would be able to use N as a downstream alternate, but N
   could not use S; therefore N would have no alternate and would
   discard the traffic, thus avoiding the micro-loop.

   As shown above, the use of either a node protecting LFA (described in
   Section 3.2) or a downstream path provides protection against micro-

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   looping in the event of node failure.  There are topologies where
   there may be either a node portecting LFA, a downstream path, both or
   neither.  A node may select either a node protecting LFA or a
   downstream path without risk of causing micro-loops in the event of
   neighbor node failure.  While a link-and-node-protecting LFA
   guarantees protection against either link or node failure, a
   downstream path provides protection only against a link failure and
   may or may not provide protection against a node failure depending on
   the protection available at the downstream node, but it cannot cause
   a micro-loop.  For example in Figure 2, if S uses N as a downstream
   path, although no looping can occur, the traffic will not be
   protected in the event of the failure of node E because N has no
   viable repair path, and it will simply discard the packet.  However,
   if N had a link and node protecting LFA or downstream path via some
   other path (not shown), then the repair may succeed.

   Since the functionality of link and node protecting LFAs is greater
   than that of link protecting downstream paths, a router SHOULD select
   a link and node protecting LFA over a link protecting downstream
   path.  If there are any destinations for which a link and node
   protecting LFA is not available, then by definition the path to all
   of those destinations from any neighbor of the computing router (S)
   must be through the node (E) being protected (otherwise there would
   be a node protecting LFA for that destination).  Consequently, if
   there exists a downstream path to the protected node as destination,
   then that downstream path may be used for all those destinations for
   which a link and node protecting LFA is not available; the existence
   of a downstream path can be determined by a single check of the
   condition Distance_opt(N, E) < Distance_opt(S, E).

   It may be desirable to find an alternate that can protect against
   other correlated failures (of which node failure is a specific
   instance).  In the general case, these are handled by shared risk
   link groups (SRLGs) where any links in the network can belong to the
   SRLG.  General SRLGs may add unacceptably to the computational
   complexity of finding a loop-free alternate.

   However, a sub-category of SRLGs is of interest and can be applied
   only during the selection of an acceptable alternate.  This sub-
   category is to express correlated failures of links that are
   connected to the same router.  For example, if there are multiple
   logical sub-interfaces on the same physical interface, such as VLANs
   on an Ethernet interface, if multiple interfaces use the same
   physical port because of channelization, or if multiple interfaces
   share a correlated failure because they are on the same line-card.
   This sub-category of SRLGs will be referred to as local-SRLGs.  A
   local-SRLG has all of its member links with one end connected to the
   same router.  Thus, router S could select a loop-free alternate which

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   does not use a link in the same local-SRLG as the primary next-hop.
   The local-SRLGs belonging to E can be protected against via node-
   protection; i.e. picking a loop-free node-protecting alternate.

   Where SRLG protection is provided, it is in the context of the
   particular OSPF or ISIS area, whose topology is used in the SPF
   computations to compute the loop-free alternates.  If an SRLG
   contains links in multiple areas, then separate SRLG-protecting
   alternates would be required in each area that is traversed by the
   affected traffic.

2.  Applicability of Described Mechanisms

   IP Fast Reroute mechanisms described in this memo cover intra-domain
   routing only, with OSPF[RFC2328] or ISIS [RFC1195] [RFC2966] as the
   IGP.  Specifically, Fast Reroute for BGP inter-domain routing is not
   part of this specification.

   Certain aspects of OSPF inter-area routing behavior explained in
   Section 6.3 and Appendix A impact the ability of the router
   calculating the backup next-hops to assess traffic trajectories.  In
   order to avoid micro-looping and ensure required coverage, certain
   constraints are applied to multi-area OSPF networks:

   a.  Loop-free alternates should not be used in the backbone area if
       there are any virtual links configured unless, for each transit
       area, there is a full mesh of virtual links between all ABRs in
       that area.  Loop-free alternates may be used in non-backbone
       areas regardless of whether there are virtual links configured.

   b.  Loop-free alternates should not be used for inter-area routes in
       an area that contains more than one alternate ABR.  [RFC3509]

   c.  Loop-free alternates should not be used for AS External routes or
       ASBR routes in a non-backbone area of a network where there
       exists an ABR that is announced as an ASBR in multiple non-
       backbone areas and there exists another ABR that is in at least
       two of the same non-backbone areas.

   d.  Loop-free alternates should not be used in a non-backbone area of
       a network for AS External routes where an AS External prefix is
       advertised with the same type of external metric by multiple
       ASBRs, which are in different non-backbone areas, with a
       forwarding address of or by one or more ASBRs with
       forwarding addresses in multiple non-backbone areas when an ABR
       exists simultaneously in two or more of those non-backbone areas.

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3.  Alternate Next-Hop Calculation

   In addition to the set of primary next-hops obtained through a
   shortest path tree (SPT) computation that is part of standard link-
   state routing functionality, routers supporting IP Fast Reroute also
   calculate a set of backup next hops that are engaged when a local
   failure occurs.  These backup next hops are calculated to provide the
   required type of protection (i.e. link-protecting and/or node-
   protecting) and to guarantee that when the expected failure occurs,
   forwarding traffic through them will not result in a loop.  Such next
   hops are called loop-free alternates or LFAs throughout this

   In general, to be able to calculate the set of LFAs for a specific
   destination D, a router needs to know the following basic pieces of

   o  Shortest-path distance from the calculating router to the
      destination (Distance_opt(S, D))

   o  Shortest-path distance from the router's IGP neighbors to the
      destination (Distance_opt(N, D))

   o  Shortest path distance from the router's IGP neighbors to itself
      (Distance_opt(N, S))

   o  Distance_opt(S, D) is normally available from the regular SPF
      calculation performed by the link-state routing protocols.
      Distance_opt(N, D) and Distance_opt(N, S) can be obtained by
      performing additional SPF calculations from the perspective of
      each IGP neighbor (i.e. considering the neighbor's vertex as the
      root of the SPT--called SPT(N) hereafter--rather than the
      calculating router's one, called SPT(S)).

   This specification defines a form of SRLG protection limited to those
   SRLGs that include a link that the calculating router is directly
   connected to.  Only that set of SRLGs could cause a local failure;
   the calculating router only computes alternates to handle a local
   failure.  Information about local link SRLG membership is manually
   configured.  Information about remote link SRLG membership may be
   dynamically obtained using [RFC4205] or [RFC4203].  Define
   SRLG_local(S) to be the set of SRLGs that include a link that the
   calculating router S is directly connected to.  Only SRLG_local(S) is
   of interest during the calculation, but the calculating router must
   correctly handle changes to SRLG_local(S) triggered by local link
   SRLG membership changes.

   In order to choose among all available LFAs that provide required

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   SRLG protection for a given destination, the calculating router needs
   to track the set of SRLGs in SRLG_local(S) that the path through a
   specific IGP neighbor involves.  To do so, each node D in the network
   topology is associated with SRLG_set(N, D), which is the set of SRLGs
   that would be crossed if traffic to D was forwarded through N. To
   calculate this set, the router initializes SRLG_set(N, N) for each of
   its IGP neighbors to be empty.  During the SPT(N) calculation, when a
   new vertex V is added to the SPT, its SRLG_set(N, V) is set to the
   union of SRLG sets associated with its parents, and the SRLG sets in
   SRLG_local(S) that are associated with the links from V's parents to
   V. The union of the set of SRLG associated with a candidate alternate
   next-hop and the SRLG_set(N, D) for the neighbor reached via that
   candidate next-hop is used to determine SRLG protection.

   The following sections provide information required for calculation
   of LFAs.  Sections Section 3.1 through Section 3.4 define different
   types of LFA conditions.  Section 3.5 describes constraints imposed
   by the IS-IS overload and OSPF stub router functionality.
   Section 3.6 defines the summarized algorithm for LFA calculation
   using the definitions in the previous sections.

3.1.  Basic Loop-free Condition

   Alternate next hops used by implementations following this
   specification MUST conform to at least the loop-freeness condition
   stated above in Inequality 1.  This condition guarantees that
   forwarding traffic to an LFA will not result in a loop after a link

   Further conditions may be applied when determining link-protecting
   and/or node-protecting alternate next-hops as described in Sections
   Section 3.2 and Section 3.3.

3.2.  Node-Protecting Alternate Next-Hops

   For an alternate next-hop N to protect against node failure of a
   primary neighbor E for destination D, N must be loop-free with
   respect to both E and D. In other words, N's path to D must not go
   through E. This is the case if Inequality 3 is true, where N is the
   neighbor providing a loop-free alternate.

         Distance_opt(N, D) < Distance_opt(N, E) + Distance_opt(E, D)

     Inequality 3: Criteria for a Node-Protecting Loop-Free Alternate

   If Distance_opt(N,D) = Distance_opt(N, E) + Distance_opt(E, D), it is
   possible that N has equal-cost paths and one of those could provide
   protection against E's node failure.  However, it is equally possible

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   that one of N's paths goes through E, and the calculating router has
   no way to influence N's decision to use it.  Therefore, it SHOULD be
   assumed that an alternate next-hop does not offer node protection if
   Inequality 3 is not met.

3.3.  Broadcast and NBMA Links

   Verification of the link-protection property of a next hop in the
   case of a broadcast link is more elaborate than for a point-to-point
   link.  This is because a broadcast link is represented as a pseudo-
   node with zero-cost links connecting it to other nodes.

   Because failure of an interface attached to a broadcast segment may
   mean loss of connectivity of the whole segment, the condition
   described for broadcast link protection is pessimistic and requires
   that the alternate is loop-free with regard to the pseudo-node.
   Consider the example in Figure 3.

                       +-----+    15
                       |  S  |--------
                       +-----+       |
                          | 5        |
                          |          |
                          | 0        |
                        /----\ 0 5 +-----+
                        | PN |-----|  N  |
                        \----/     +-----+
                           | 0        |
                           |          | 8
                           | 5        |
                        +-----+ 5  +-----+
                        |  E  |----|  D  |
                        +-----+    +-----+

           Figure 3: Loop-Free Alternate that is Link-Protecting

   In Figure 3, N offers a loop-free alternate which is link-protecting.
   If the primary next-hop uses a broadcast link, then an alternate
   SHOULD be loop-free with respect to that link's pseudo-node (PN) to
   provide link protection.  This requirement is described in
   Inequality 4 below.

              D_opt(N, D) < D_opt(N, PN) + D_opt(PN, D)

   Inequality 4: Loop-Free Link-Protecting Criterion for Broadcast Links

   Because the shortest path from the pseudo-node goes through E, if a
   loop-free alternate from a neighbor N is node-protecting, the

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   alternate will also be link-protecting unless the router S can only
   reach the alternate neighbor N via the same pseudo-node.  Since this
   is the only case for which a node-protecting LFA is not link-
   protecting, this implies that for point-to-point interfaces, an LFA
   that is node-protecting is always link-protecting.  Because S can
   direct the traffic away from the shortest path to use the alternate
   N, traffic might pass through the same broadcast link as it would
   when S sent the traffic to the primary E. Thus, an LFA from N that is
   node-protecting is not automatically link-protecting for a broadcast
   or NBMA link.

   To obtain link protection, it is necessary both that the path from
   the selected alternate next-hop does not traverse the link of
   interest and that the link used from S to reach that alternate next-
   hop is not the link of interest.  The latter can only occur with non-
   point-to-point links.  Therefore, if the primary next-hop is across a
   broadcast or NBMA interface, it is necessary to consider link
   protection during the alternate selection.  To clarify, consider the
   topology in Figure 3.  For N to provide link-protection, it is first
   necessary that N's shortest path to D does not traverse the pseudo-
   node PN.  Second, it is necessary that the alternate next-hop
   selected by S does not traverse PN.  In this example, S's shortest
   path to N is via the pseudo-node.  Thus, to obtain link-protection, S
   must find a next-hop to N (the point-to-point link from S to N in
   this example) that avoids the pseudo-node PN.

   Similar consideration of the link from S to the selected alternate
   next-hop as well as the path from the selected alternate next-hop is
   also necessary for SRLG protection.  S's shortest path to the
   selected neighbor N may not be acceptable as an alternate next-hop to
   provide SRLG protection, even if the path from N to D can provide
   SRLG protection.

3.4.  ECMP and Alternates

   With equal-cost multi-path, a prefix may have multiple primary next-
   hops that are used to forward traffic.  When a particular primary
   next-hop fails, alternate next-hops should be used to preserve the
   traffic.  These alternate next-hops may themselves also be primary
   next-hops, but need not be.  Other primary next-hops are not
   guaranteed to provide protection against the failure scenarios of

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                           20 L1      L3  3
                       [ N ]----[ S ]--------[ E3 ]
                         |        |            |
                         |      5 | L2         |
                      20 |        |            |
                         |    ---------        | 2
                         |  5 |       | 5      |
                         |  [ E1 ]  [ E2 ]-----|
                         |     |       |
                         | 10  |    10 |
                         |---[ A ]   [ B ]
                              |        |
                            2 |--[ D ]-| 2

     Figure 4: ECMP where Primary Next-Hops Provide Limited Protection

   In Figure 4 S has three primary next-hops to reach D; these are L2 to
   E1, L2 to E2 and L3 to E3.  The primary next-hop L2 to E1 can obtain
   link and node protection from L3 to E3, which is one of the other
   primary next-hops; L2 to E1 cannot obtain link protection from the
   other primary next-hop L2 to E2.  Similarly, the primary next-hop L2
   to E2 can only get node protection from L2 to E1 and can only get
   link protection from L3 to E3.  The third primary next-hop L3 to E3
   can obtain link and node protection from L2 to E1 and from L2 to E2.
   It is possible for both the primary next-hop L2 to E2 and the primary
   next-hop L2 to E1 to obtain an alternate next-hop that provides both
   link and node protection by using L1.

   Alternate next-hops are determined for each primary next-hop
   separately.  As with alternate selection in the non-ECMP case, these
   alternate next-hops should maximize the coverage of the failure

3.5.  Interactions with ISIS Overload, RFC 3137 and Costed Out Links

   As described in [RFC3137], there are cases where it is desirable not
   to have a router used as a transit node.  For those cases, it is also
   desirable not to have the router used on an alternate path.

   For computing an alternate, a router MUST NOT use an alternate next-
   hop that is along a link whose cost or reverse cost is LSInfinity
   (for OSPF) or the maximum cost (for ISIS) or which has the overload
   bit set (for ISIS).  For a broadcast link, the reverse cost
   associated with a potential alternate next-hop is the cost towards
   the pseudo-node advertised by the next-hop router.  For point-to-
   point links, if a specific link from the next-hop router cannot be
   associated with a particular link, then the reverse cost considered
   is that of the minimum cost link from the next-hop router back to S.

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   In the case of OSPF, if all links from router S to a neighbor N_i
   have a reverse cost of LSInfinity, then router S MUST NOT use N_i as
   an alternate.

   Similarly in the case of ISIS, if N_i has the overload bit set, then
   S MUST NOT consider using N_i as an alternate.

   This preserves the desired behavior of diverting traffic away from a
   router which is following [RFC3137] and it also preserves the desired
   behavior when an operator sets the cost of a link to LSInfinity for
   maintenance which is not permitting traffic across that link unless
   there is no other path.

   If a link or router which is costed out was the only possible
   alternate to protect traffic from a particular router S to a
   particular destination, then there should be no alternate provided
   for protection.

3.5.1.  Interactions with ISIS Link Attributes

   [RFC5029] describes several flags whose interactions with LFAs needs
   to be defined.  A router SHOULD NOT specify the "local protection
   available" flag as a result of having LFAs.  A router SHOULD NOT use
   an alternate next-hop that is along a link for which the link has
   been advertised with the attribute "link excluded from local
   protection path" or with the attribute "local maintenance required".

3.6.  Selection Procedure

   A router supporting this specification SHOULD attempt to select at
   least one loop-free alternate next-hop for each primary next-hop used
   for a given prefix.  A router MAY decide to not use an available
   loop-free alternate next-hop.  A reason for such a decision might be
   that the loop-free alternate next-hop does not provide protection for
   the failure scenario of interest.

   The alternate selection should maximize the coverage of the failure

   When calculating alternate next-hops, the calculating router S
   applies the following rules.

   1.  S SHOULD select a loop-free node-protecting alternate next-hop,
       if one is available.  If no loop-free node-protecting alternate
       is available, then S MAY select a loop-free link-protecting

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   2.  If S has a choice between a loop-free link-protecting node-
       protecting alternate and a loop-free node-protecting alternate
       which is not link-protecting, S SHOULD select a loop-free node-
       protecting alternate which is also link-protecting.  This can
       occur as explained in Section 3.3.

   3.  If S has multiple primary next-hops, then S SHOULD select as a
       loop-free alternate either one of the other primary next-hops or
       a loop-free node-protecting alternate if available.  If no loop-
       free node-protecting alternate is available and no other primary
       next-hop can provide link-protection, then S SHOULD select a
       loop-free link-protecting alternate.

   4.  Implementations SHOULD support a mode where other primary next-
       hops satisfying the basic loop-free condition and providing at
       least link or node protection are preferred over any non-primary
       alternates.  This mode is provided to allow the administrator to
       preserve traffic patterns based on regular ECMP behavior.

   5.  Implementations considering SRLGs MAY use SRLG-protection to
       determine that a node-protecting or link-protecting alternate is
       not available for use.

   Following the above rules maximizes the level of protection and use
   of primary (ECMP) next-hops.

   Each next-hop is associated with a set of non-mutually-exclusive
   characteristics based on whether it is used as a primary next-hop to
   a particular destination D, and the type of protection it can provide
   relative to a specific primary next-hop E:

   Primary Path -  The next-hop is used by S as primary.

   Loop-Free Node-Protecting Alternate -  This next-hop satisfies
      Inequality 1 and Inequality 3.  The path avoids S, S's primary
      neighbor E, and the link from S to E.

   Loop-Free Link-Protecting Alternate -   This next-hop satisfies
      Inequality 1 but not Inequality 3.  If the primary next-hop uses a
      broadcast link, then this next-hop satisfies Inequality 4.

   An alternate path may also provide none, some or complete SRLG
   protection as well as node and link or link protection.  For
   instance, a link may belong to two SRLGs G1 and G2.  The alternate
   path might avoid other links in G1 but not G2, in which case the
   alternate would only provide partial SRLG protection.

   Below is an algorithm that can be used to calculate loop-free

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   alternate next-hops.  The algorithm is given for informational
   purposes and implementations are free to use any other algorithm as
   long as it satisfies the rules described above.

   The following procedure describes how to select an alternate next-
   hop.  The procedure is described to determine alternate next-hops to
   use to reach each router in the topology.  Prefixes that are
   advertised by a single router can use the alternate next-hop computed
   for the router to which they are attached.  The same procedure can be
   used to reach a prefix that is advertised by more than one router
   when the logical topological transformation described in Section 6.1
   is used.

   S is the computing router.  S has neighbors N_1 to N_j.  A candidate
   next-hop is indicated by (outgoing link, neighbor) and the outgoing
   link must be bidirectionally connected, as is determined by the IGP.
   The candidate next-hops of S are enumerated as H_1 through H_k.
   Recall that S may have multiple next hops over different interfaces
   to a neighbor. refers to the outgoing link of that next-hop
   and H_i.neighbor refers to the neighbor of that nexthop.

   For a particular destination router D, let S have already computed
   D_opt(S, D), and for each neighbor N_i, D_opt(N_i, D), D_opt(N_i, S),
   and D_opt(N_i, N_j), the distance from N_i to each other neighbor
   N_j, and the set of SRLGs traversed by the path D_opt(N_i, D).  S
   should follow the below procedure for every primary next-hop selected
   to reach D. This set of primary next-hops is represented P_1 to P_p.
   This procedure finds the alternate next-hop(s) for P_i.

   First, initialize the alternate information for P_i as follows:

      P_i.alt_next_hops = {}
      P_i.alt_type = NONE
      P_i.alt_link-protect = FALSE
      P_i.alt_node-protect = FALSE
      P_i.alt_srlg-protect = {}

   For each candidate next-hop H_h,

   1.   Initialize variables as follows:

           cand_type = NONE
           cand_link-protect = FALSE
           cand_node-protect = FALSE
           cand_srlg-protect = {}

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   2.   If H_h is P_i, skip it and continue to the next candidate next-

   3.   If is administratively allowed to be used as an

           and the cost of is less than the maximum,
           and the reverse cost of H_h is less than the maximum,
           and H_h.neighbor is not overloaded (for ISIS),
           and is bi-directional,

        then H_h can be considered as an alternate.  Otherwise, skip it
        and continue to the next candidate next-hop.

   4.   If D_opt( H_h.neighbor, D) >= D_opt( H_h.neighbor, S) + D_opt(S,
        D), then H_h is not loop-free.  Skip it and continue to the next
        candidate next-hop.

   5.   cand_type = LOOP-FREE.

   6.   If H_h is a primary next-hop, set cand_type to PRIMARY.

   7.   If is not, set cand_link-protect to TRUE.

   8.   If D_opt(H_h.neighbor, D) < D_opt(H_h.neighbor, P_i.neighbor) +
        D_opt(P_i.neighbor, D), set cand_node-protect to TRUE.

   9.   If the router considers SRLGs, then set the cand_srlg-protect to
        the set of SRLGs traversed on the path from S via to
        P_i.neighbor.  Remove the set of SRLGs to which H_h belongs from
        cand_srlg-protect.  Remove from cand_srlg-protect the set of
        SRLGs traversed on the path from H_h.neighbor to D. Now
        cand_srlg-protect holds the set of SRLGs to which P_i belongs
        and that are not traversed on the path from S via H_h to D.

   10.  If cand_type is PRIMARY, the router prefers other primary next-
        hops for use as the alternate, and the P_i.alt_type is not
        PRIMARY, goto Step 20.

   11.  If cand_type is not PRIMARY, P_i.alt_type is PRIMARY and the
        router prefers other primary next-hops for use as the alternate,
        then continue to the next candidate next-hop

   12.  If cand_node-protect is TRUE and P_i.alt_node-protect is FALSE,
        goto Paragraph 20.

   13.  If cand_link-protect is TRUE and P_i.alt_link-protect is FALSE,
        goto Step 20.

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   14.  If cand_srlg-protect has a better set of SRLGs than
        P_i.alt_srlg-protect, goto Step 20.

   15.  If cand_srlg-protect is different from P_i.alt_srlg-protect,
        then select between H_h and P_i.alt_next_hops based upon
        distance, IP addresses, or any router-local tie-breaker.  If H_h
        is preferred, then goto Step 20.  If P_i.alt_next_hops is
        preferred, skip H_h and continue to the next candidate next-hop.

   16.  If D_opt(H_h.neighbor, D) < D_opt(P_i.neighbor, D) and
        D_opt(P_i.alt_next_hops, D) >= D_opt(P_i.neighbor, D), then H_h
        is a downstream alternate and P_i.alt_next_hops is simply an
        LFA.  Prefer H_h and goto Step 20.

   17.  Based upon the alternate types, the alternate distances, IP
        addresses, or other tie-breakers, decide if H_h is preferred to
        P_i.alt_next_hops.  If so, goto Step 20.

   18.  Decide if P_i.alt_next_hops is preferred to H_h.  If so, then
        skip H_h and continue to the next candidate next-hop.

   19.  Add H_h into P_i.alt_next_hops.  Set P_i.alt_type to the better
        type of H_h.alt_type and P_i.alt_type.  Continue to the next
        candidate next-hop.

   20.  Replace the P_i alternate next-hop set with H_h as follows:

           P_i.alt_next_hops = {H_h}
           P_i.alt_type = cand_type
           P_i.alt_link-protect = cand_link-protect
           P_i.alt_node-protect = cand_node-protect
           P_i.alt_srlg-protect = cand_srlg-protect

        Continue to the next candidate next-hop.

3.7.  LFA types and Trade-offs

   LFAs can provide different amounts of protection and the decision
   about which type to prefer is dependent upon network topology and
   other techniques in use in the network.  This section describes the
   different protection levels and the trade-offs associated with each.

   1.  Primary Next-hop: When there are equal-cost primary next-hops,
       using one as an alternate is guaranteed to not cause micro-loops
       involving S. Traffic flows across the paths that the network will
       converge to, but congestion may be experienced on the primary
       paths since traffic is sent across fewer.  All primary next-hops
       are downstream paths.

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   2.  Downstream Paths: A downstream path, unlike an LFA, is guaranteed
       not to cause a micro-loop involving S regardless of the actual
       failure detected.  However, the expected coverage of such
       alternates in a network is expected to be poor.  All downstream
       paths are LFAs.

   3.  LFA: An LFA can have good coverage of a network, depending on
       topology.  However, it is possible to get micro-loops involving S
       if a more severe failure occurs than is protected against.

   The different types of protection are abbreviated as LP (link-
   protecting), NP (node-protecting) and SP (SRLG-protecting).

   a.  LP, NP, and SP: If such an alternate exists, it gives protection
       against all failures.

   b.  LP and NP only: Many networks may handle SRLG failures via
       another method or may focus on node and link failures as being
       more common.

   c.  LP only: A network may handle node failures via a high
       availability technique and be concerned primarily about
       protecting the more common link failure case.

   d.  NP only: These only exist on interfaces that aren't point-to-
       point.  If link protection is handled in a different layer, then
       an NP alternate may be acceptable.

3.8.  A Simplification: Per-Next-Hop LFAs

   It is possible to simplify the computation and use of LFAs when
   solely link protection is desired by considering and computing only
   one link-protecting LFA for each next-hop connected to the router.
   All prefixes that use that next-hop as a primary will use the LFA
   computed for that next-hop as its LFA.

   Even a prefix with multiple primary next-hops will have each primary
   next-hop protected individually by the primary next-hop's associated
   LFA.  That associated LFA might or might not be another of the
   primary next-hops of the prefix.

   This simplification may reduce coverage in a network.  In addition to
   limiting protection for multi-homed prefixes (see Section 6.1), the
   computation per next-hop may also not find an LFA when one could be
   found for some of the prefixes that use that next-hop.

   For example, consider Figure 4 where S has 3 ECMP next-hops, E1, E2,
   and E3 to reach D. For the prefix D, E3 can give link protection for

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   the next-hops E1 and E2; E1 and E2 can give link protection for the
   next-hops E3.  However, if one uses this simplification to compute
   LFAs for E1, E2 and E3 individually, there is no link-protecting LFA
   for E1.  E3 and E2 can protect each other.

4.  Using an Alternate

   If an alternate next-hop is available, the router redirects traffic
   to the alternate next-hop in case of a primary next-hop failure as

   When a next-hop failure is detected via a local interface failure or
   other failure detection mechanisms (see
   [I-D.ietf-rtgwg-ipfrr-framework]), the router SHOULD:

   1.  Remove the primary next-hop associated with the failure.

   2.  Install the loop-free alternate calculated for the failed next-
       hop if it is not already installed (e.g. the alternate is also a
       primary next-hop).

   Note that the router MAY remove other next-hops if it believes (via
   SRLG analysis) that they may have been affected by the same failure,
   even if it is not visible at the time of failure detection.

   The alternate next-hop MUST be used only for traffic types which are
   routed according to the shortest path.  Multicast traffic is
   specifically out of scope for this specification.

4.1.  Terminating Use of Alternate

   A router MUST limit the amount of time an alternate next-hop is used
   after the primary next-hop has become unavailable.  This ensures that
   the router will start using the new primary next-hops.  It ensures
   that all possible transient conditions are removed and the network
   converges according to the deployed routing protocol.

   There are techniques available to handle the micro-forwarding loops
   that can occur in a networking during convergence.

   A router that implements [I-D.ietf-rtgwg-microloop-analysis] SHOULD
   follow the rules given there for terminating the use of an alternate.

   A router that implements [I-D.francois-ordered-fib] SHOULD follow the
   rules given there for terminating the use of an alternate.

   It is desirable to avoid micro-forwarding loops involving S. An

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   example illustrating the problem is given in Figure 5.  If the link
   from S to E fails, S will use N1 as an alternate and S will compute
   N2 as the new primary next-hop to reach D. If S starts using N2 as
   soon as S can compute and install its new primary, it is probable
   that N2 will not have yet installed its new primary next-hop.  This
   would cause traffic to loop and be dropped until N2 has installed the
   new topology.  This can be avoided by S delaying its installation and
   leaving traffic on the alternate next-hop.

                          |  N2 |--------   |
                          +-----+   1   |  \|/
                              |         |
                              |     +-----+    @@>  +-----+
                              |     |  S  |---------|  N1 |
                           10 |     +-----+   10    +-----+
                              |        |               |
                              |      1 |    |          |
                              |        |   \|/    10   |
                              |     +-----+            |  |
                              |     |  E  |            | \|/
                              |     +-----+            |
                              |        |               |
                              |      1 |  |            |
                              |        | \|/           |
                              |    +-----+             |
                              |----|  D  |--------------

      Figure 5: Example where Continued Use of Alternate is Desirable

   This is an example of a case where the new primary is not a loop-free
   alternate before the failure and therefore may have been forwarding
   traffic through S. This will occur when the path via a previously
   upstream node is shorter than the the path via a loop-free alternate
   neighbor.  In these cases, it is useful to give sufficient time to
   ensure that the new primary neighbor and other nodes on the new
   primary path have switched to the new route.

   If the newly selected primary was loop-free before the failure, then
   it is safe to switch to that new primary immediately; the new primary
   wasn't dependent on the failure and therefore its path will not have

   Given that there is an alternate providing appropriate protection and
   while the assumption of a single failure holds, it is safe to delay
   the installation of the new primaries; this will not create
   forwarding loops because the alternate's path to the destination is

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   known to not go via S or the failed element and will therefore not be
   affected by the failure.

   An implementation SHOULD continue to use the alternate next-hops for
   packet forwarding even after the new routing information is available
   based on the new network topology.  The use of the alternate next-
   hops for packet forwarding SHOULD terminate:

   a.  if the new primary next-hop was loop-free prior to the topology
       change, or

   b.  if a configured hold-down, which represents a worst-case bound on
       the length of the network convergence transition, has expired, or

   c.  if notification of an unrelated topological change in the network
       is received.

5.  Requirements on LDP Mode

   Since LDP [RFC5036] traffic will follow the path specified by the
   IGP, it is also possible for the LDP traffic to follow the loop-free
   alternates indicated by the IGP.  To do so, it is necessary for LDP
   to have the appropriate labels available for the alternate so that
   the appropriate out-segments can be installed in the forwarding plane
   before the failure occurs.

   This means that a Label Switched Router (LSR) running LDP must
   distribute its labels for the FECs it can provide to all its
   neighbors, regardless of whether or not they are upstream.
   Additionally, LDP must be acting in liberal label retention mode so
   that the labels which correspond to neighbors that aren't currently
   the primary neighbor are stored.  Similarly, LDP should be in
   downstream unsolicited mode, so that the labels for the FEC are
   distributed other than along the SPT.

   If these requirements are met, then LDP can use the loop-free
   alternates without requiring any targeted sessions or signaling
   extensions for this purpose.

6.  Routing Aspects

6.1.  Multi-Homed Prefixes

   An SPF-like computation is run for each topology, which corresponds
   to a particular OSPF area or ISIS level.  The IGP needs to determine
   loop-free alternates to multi-homed routes.  Multi-homed routes occur

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   for routes obtained from outside the routing domain by multiple
   routers, for subnets on links where the subnet is announced from
   multiple ends of the link, and for routes advertised by multiple
   routers to provide resiliency.

   Figure 6 demonstrates such a topology.  In this example, the shortest
   path to reach the prefix p is via E. The prefix p will have the link
   to E as its primary next-hop.  If the alternate next-hop for the
   prefix p is simply inherited from the router advertising it on the
   shortest path to p, then the prefix p's alternate next-hop would be
   the link to C. This would provide link protection, but not the node
   protection that is possible via A.

                      5   +---+  8   +---+  5  +---+
                    ------| S |------| A |-----| B |
                    |     +---+      +---+     +---+
                    |       |                    |
                    |     5 |                  5 |
                    |       |                    |
                  +---+ 5 +---+   5       7    +---+
                  | C |---| E |------ p -------| F |
                  +---+   +---+                +---+

                       Figure 6: Multi-homed prefix

   To determine the best protection possible, the prefix p can be
   treated in the SPF computations as a node with uni-directional links
   to it from those routers that have advertised the prefix.  Such a
   node need never have its links explored, as it has no out-going

   If there exist multiple multi-homed prefixes that share the same
   connectivity and the difference in metrics to those routers, then a
   single node can be used to represent the set.  For instance, if in
   Figure 6 there were another prefix X that was connected to E with a
   metric of 1 and to F with a metric of 3, then that prefix X could use
   the same alternate next-hop as was computed for prefix p.

   A router SHOULD compute the alternate next-hop for an IGP multi-homed
   prefix by considering alternate paths via all routers that have
   announced that prefix.

6.2.  ISIS

   The applicability and interactions of LFAs with multi-topology ISIS
   [RFC5120] is out of scope for this specification.

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6.3.  OSPF

   OSPF introduces certain complications because it is possible for the
   traffic path to exit an area and then re-enter that area.  This can
   occur whenever a router considers the same route from multiple areas.
   There are several cases where issues such as this can occur.  They
   happen when another area permits a shorter path to connect two ABRs
   than is available in the area where the LFA has been computed.  To
   clarify, an example topology is given in Appendix A.

   a.  Virtual Links: These allow paths to leave the backbone area and
       traverse the transit area.  The path provided via the transit
       area can exit via any ABR.  The path taken is not the shortest
       path determined by doing an SPF in the backbone area.

   b.  Alternate ABR[RFC3509]: When an ABR is not connected to the
       backbone, it considers the inter-area summaries from multiple
       areas.  The ABR A may determine to use area 2 but that path could
       traverse another alternate ABR B that determines to use area 1.
       This can lead to scenarios similar to that illustrated in
       Figure 7.

   c.  ASBR Summaries: An ASBR may itself be an ABR and can be announced
       into multiple areas.  This presents other ABRs with a decision as
       to which area to use.  This is the example illustrated in
       Figure 7.

   d.  AS External Prefixes: A prefix may be advertised by multiple
       ASBRs in different areas and/or with multiple forwarding
       addresses that are in different areas, which are connected via at
       least one common ABR.  This presents such ABRs with a decision as
       to which area to use to reach the prefix.

   Loop-free alternates should not be used in an area where one of the
   above issues affects that area.

6.3.1.  OSPF External Routing

   When a forwarding address is set in an OSPF AS-external LSA, all
   routers in the network calculate their next-hops for the external
   prefix by doing a lookup for the forwarding address in the routing
   table, rather than using the next-hops calculated for the ASBR.  In
   this case, the alternate next-hops SHOULD be computed by selecting
   among the alternate paths to the forwarding link(s) instead of among
   alternate paths to the ASBR.

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6.3.2.  OSPF Multi-Topology

   The applicabilty and interactions of LFAs with multi-topology OSPF
   [RFC4915] [I-D.ietf-ospf-mt-ospfv3] is out of scope for this

6.4.  BGP Next-Hop Synchronization

   Typically BGP prefixes are advertised with AS exit routers router-id
   as the BGP next-hop, and AS exit routers are reached by means of IGP
   routes.  BGP resolves its advertised next-hop to the immediate next-
   hop by potential recursive lookups in the routing database.  IP Fast-
   Reroute computes the alternate next-hops to all IGP destinations,
   which include alternate next-hops to the AS exit router's router-id.
   BGP simply inherits the alternate next-hop from IGP.  The BGP
   decision process is unaltered; BGP continues to use the IGP optimal
   distance to find the nearest exit router.  MBGP routes do not need to
   copy the alternate next hops.

   It is possible to provide ASBR protection if BGP selected a set of
   BGP next-hops and allowed the IGP to determine the primary and
   alternate next-hops as if the BGP route were a multi-homed prefix.
   This is for future study.

6.5.  Multicast Considerations

   Multicast traffic is out of scope for this specification of IP Fast-
   Reroute.  The alternate next-hops SHOULD NOT be used for multicast
   RPF checks.

7.  Security Considerations

   The mechanism described in this document does not modify any routing
   protocol messages, and hence no new threats related to packet
   modifications or replay attacks are introduced.  Traffic to certain
   destinations can be temporarily routed via next-hop routers that
   would not be used with the same topology change if this mechanism
   wasn't employed.  However, these next-hop routers can be used anyway
   when a different topological change occurs, and hence this can't be
   viewed as a new security threat.

   In LDP, the wider distribution of FEC label information is still to
   neighbors with whom a trusted LDP session has been established.  This
   wider distribution and the recommendation of using liberal label
   retention mode are believed to have no significant security impact.

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8.  IANA Considerations

   This document requires no IANA considerations.

9.  Acknowledgements

   The authors would like to thank Joel Halpern, Mike Shand, Stewart
   Bryant, and Stefano Previdi for their assistance and useful review.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC2740]  Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
              RFC 2740, December 1999.

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

10.2.  Informative References

              Francois, P., "Loop-free convergence using oFIB",
              draft-francois-ordered-fib-02 (work in progress),
              October 2006.

              Mirtorabi, S. and A. Roy, "Multi-topology routing in
              OSPFv3 (MT-OSPFv3)", draft-ietf-ospf-mt-ospfv3-03 (work in
              progress), July 2007.

              Ferguson, D., "OSPF for IPv6",
              draft-ietf-ospf-ospfv3-update-18 (work in progress),
              November 2007.

              Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              draft-ietf-rtgwg-ipfrr-framework-08 (work in progress),
              February 2008.

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              Zinin, A., "Analysis and Minimization of Microloops in
              Link-state Routing Protocols",
              draft-ietf-rtgwg-microloop-analysis-01 (work in progress),
              October 2005.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, December 1990.

   [RFC2966]  Li, T., Przygienda, T., and H. Smit, "Domain-wide Prefix
              Distribution with Two-Level IS-IS", RFC 2966,
              October 2000.

   [RFC3137]  Retana, A., Nguyen, L., White, R., Zinin, A., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 3137,
              June 2001.

   [RFC3509]  Zinin, A., Lindem, A., and D. Yeung, "Alternative
              Implementations of OSPF Area Border Routers", RFC 3509,
              April 2003.

   [RFC4203]  Kompella, K. and Y. Rekhter, "OSPF Extensions in Support
              of Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 4203, October 2005.

   [RFC4205]  Kompella, K. and Y. Rekhter, "Intermediate System to
              Intermediate System (IS-IS) Extensions in Support of
              Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 4205, October 2005.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, June 2007.

   [RFC5029]  Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
              Attribute Sub-TLV", RFC 5029, September 2007.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

Appendix A.  OSPF Example Where LFA Based on Local Area Topology is

   This appendix provides an example scenario where the local area
   topology does not suffice to determine that an LFA is available.  As
   described in Section 6.3, one problem scenario is for ASBR summaries

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   where the ASBR is available in two areas via intra-area routes and
   there is at least one ABR or alternate ABR that is in both areas.
   The following Figure 7 illustrates this case.

                     [ F ]-----------[ C ]
                       |               |
                       |               | 5
                    20 |          5    |     1
                       |   [ N ]-----[ A ]*****[ F ]
                       |     |         #         *
                       |  40 |         # 50      *  2
                       |     |    5    #    2    *
                       |   [ S ]-----[ B ]*****[ G ]
                       |     |         *
                       |   5 |         * 15
                       |     |         *
                       |   [ E ]     [ H ]
                       |     |         *
                       |   5 |         * 10**
                       |     |         *
                       |---[ X ]----[ ASBR ]

                       ----  Link in Area 1
                       ****  Link in Area 2
                       ####  Link in Backbone Area 0

      Figure 7: Topology with Multi-area ASBR Causing Area Transiting

   In Figure 7, the ASBR is also an ABR and is announced into both area
   1 and area 2.  A and B are both ABRs that are also connected to the
   backbone area.  S determines that N can provide a loop-free alternate
   to reach the ASBR.  N's path goes via A. A also sees an intra-area
   route to ASBR via Area 2; the cost of the path in area 2 is 30, which
   is less than 35, the cost of the path in area 1.  Therefore, A uses
   the path from area 2 and directs traffic to F. The path from F in
   area 2 goes to B. B is also an ABR and learns the ASBR from both
   areas 1 and area 2; B's path via area 1 is shorter (cost 20) than B's
   path via area 2 (cost 25).  Therefore, B uses the path from area 1
   that connects to S.

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Authors' Addresses

   Alia K. Atlas (editor)


   Alex Zinin (editor)
   701 E Middlefield Rd.
   Mountain View, CA  94043


   Raveendra Torvi
   FutureWei Technologies Inc.
   1700 Alma Dr. Suite 100
   Plano, TX  75075


   Gagan Choudhury
   200 Laurel Avenue, Room D5-3C21
   Middletown, NJ  07748

   Phone: +1 732 420-3721

   Christian Martin
   iPath Technologies


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   Brent Imhoff
   Juniper Networks
   1194 North Mathilda
   Sunnyvale, CA  94089

   Phone: +1 314 378 2571

   Don Fedyk
   Nortel Networks
   600 Technology Park
   Billerica, MA  01821

   Phone: +1 978 288 3041

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Full Copyright Statement

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