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Internet Draft                               Alia Atlas, Ed (Avici Systems)
Expires: March 2005


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

                draft-ietf-rtgwg-ipfrr-spec-base-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

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Abstract


   This document describes the use of loop-free alternates to provide
   local protection for IP unicast and/or LDP traffic in the event of a
   single link or node failure.  When a topology change occurs, a router
   S determines for each prefix an alternate next-hop which can be used
   if the primary next-hop fails.  An acceptable alternate next-hop must
   be a loop-free alternate, which goes to a neighbor whose shortest
   path to the prefix does not go back through the router S.





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Contents

  1  Introduction  .................................................  2
    1.1 Failure Scenarios  .........................................  4
  2  Alternate Next-Hop Calcuation  ................................  6
    2.1  Basic Loop-Free Condition  ................................  6
    2.2  Node-Protecting Alternate Next-Hops   .....................  6
    2.3  Broadcast and NBMA Links  .................................  6
    2.4  Interactions wtih ISIS Overload, RFC 3137
         and Costed Out Links  .....................................  7
    2.5  Selection Procedure  ......................................  8
  3  Using an Alternate  ...........................................  9
    3.1  Terminating Use of Alternate  .............................  9
  4  Requirements on LDP Mode  ..................................... 11
  5  Routing Aspects  .............................................. 11
  5.1  Multiple-Region Routing  .................................... 12
  5.1.1  Inheriting Alternate Next-Hops with One Primary Neighbor  . 14
  5.1.2  OSPF Inter-Area Routes  ................................... 14
  5.1.3  OSPF Inter-Area Routes  ................................... 15
  5.1.4  ISIS Multi-Level Routing  ................................. 15
  5.2  OSPF Virtual Links  ......................................... 15
  5.3  BGP Next-Hop Synchronization  ............................... 16
  5.4  Multicast Considerations  ................................... 16
  6  Security Considerations  ...................................... 16
  7  Full Copyright Statement  ..................................... 16
  8  References  ................................................... 17
  9  Authors Information  .......................................... 17
  10 Editor's Information  ......................................... 18





1. Introduction

   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 seconds; the application traffic
   may be sensitive to losses greater than 10s of milliseconds.

   As discussed in [FRAMEWORK], minimizing traffic loss requires a
   mechanism for the router adjacent to a failure 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



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   used in this specification is given in [FRAMEWORK].

   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 seconds.

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

                         Figure 1: Basic Topology

   The goal of IP Fast-Reroute is to reduce that traffic convergence
   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.

   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



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   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
   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 topology dependent.

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


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

                      Equation 1: Loop-Free Criterion


   A sub-set of loop-free alternate are downstream paths which must meet
   the more restrictive condition of


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

                   Equation 2: Downstream Path Criterion


1.1 Failure Scenarios

   The alternate next-hop can protect against a single link failure, a
   single node failure, or both.

   If only link protection is provided and the node fails, it is
   possible for traffic using the alternates to loop. 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 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.

   Such a scenario may be a concern if node failure is not otherwise
   protected against.  Selection of only downstream paths as alternates
   will ensure this does not occur, but such a restriction can severely



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   limit the coverage of alternates.


                                <@@@
                          @@@>
                   +-----+       +-----+
                   |  S  |-------|  N  |
                   +-+---+   5   +-----+
                     |              |
                     | 5          5 |  |
                  |  |              | \|/
                 \|/ |              |
                     |    +-----+   |
                     +----|  E  |---+
                          +--+--+
                             |
                             |
                             | 10
                             |
                          +--+--+
                          |  D  |
                          +-----+


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

   It may be desirable to find an alternate which 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 which 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
   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.




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2. Alternate Next-Hop Calculation

   To support IP Fast-Reroute, a router must be able to determine if a
   next-hop will provide a loop-free alternate before the router
   installs that next-hop as an alternate.  That next-hop must go to a
   loop-free neighbor.

   To do this computation, a router could run an SPF from the
   perspective of each of its neighbors as well as from its own
   perspective.  This provides the router with all the information
   necessary to test the equations given is this specification.

2.1 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 Equation 1.  Further conditions may be applied when
   determining link-protecting and/or node-protecting alternate next-
   hops as described in Sections 2.2 and 2.3.

2.2 Node-Protecting Alternate Next-Hops

   For an alternate next-hop to protect against node failure, the
   alternate next-hop MUST be loop-free with respect to the primary
   neighbor E and the destination.

   An alternate will be node-protecting if it doesn't go through the
   same primary neighbor as the primary next-hop.  This is the case if
   Equation 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)

      Equation 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 the neighbor may have equal-cost paths and one of those
   could provide a loop-free node-protecting alternate.  However, the
   decision as to which of equal-cost paths a router will use is a
   router-local decision.  Therefore, a router MUST assume that an
   alternate next-hop does not offer node protection if Equation 3 is
   not met.

2.3 Broadcast and NBMA Links

   The computation for link-protection is a bit more complicated for



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   broadcast links.  In an SPF computation, a broadcast links is
   represented as a pseudo-node with links of 0 cost exiting the
   pseudo-node.  For an alternate to be considered link-protecting, it
   must be loop-free with regard to the pseudo-node.  Consider the
   example in Figure 3.

                         +-----+    15
                         |  S  |-------
                         +-----+      |
                            | 5       |
                          /----\ 5 +-----+
                          | PN |----|  N  |
                          \----/   +-----+
                             |          |
                             | 5        | 2
                             |          |
                          +-----+ 5  +-----+
                          |  E  |----|  D  |
                          +-----+    +-----+
         Figure 3: Loop-Free Alternate that isn't 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 must
   be loop-free with respect to that link's pseudo-node to provide link
   protection.  This requirement is described in Equation 4 below.


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

    Equation 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
   alternate will also be link-protecting unless the router S can only
   reach the neighbor N via the same pseudo-node.  This can occur
   because S will direct traffic away from the shortest path to use an
   alternate.


2.4 Interactions with ISIS Overload, RFC 3137 and Costed Out Links

   As described in RFC 3137, 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 consider diverting from
   the SPF tree along a link whose reverse cost is LSInfinity (for OSPF)



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   or whose router has the overload bit set (for ISIS).

   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 consider
   using 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 RFC 3137 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 will be no alternate provided for
   protection.


2.5 Selection Procedure

   A router supporting this specification SHOULD select a 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 selection should maximize the failure cases which can be
   protected against.

   The selection procedure depends on whether S has a single primary
   neighbor or multiple primary neighbors.  A node S is defined to have
   a single primary neighbor only if there are no equal cost paths that
   go through any other neighbor; i.e., a node S cannot be considered to
   have a single primary neighbor simply because S does not support
   ECMP.

   If S has a single primary neighbor, then S SHOULD select a loop-free
   node-protecting alternate next-hop, if one is available.  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 2.3.  If no loop-free node-protecting alternate is available,
   then S MAY select a loop-free link-protecting alternate.



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   If S has multiple primary neighbors, then S SHOULD select as a loop-
   free alternate either one of the other primary next-hops or a loop-
   free node-protecting alternate.  S MAY select a loop-free link-
   protecting alternate.

   Each next-hop can be categorized as to the type of alternate it can
   provide to a particular destination D from router S for a particular
   primary next-hop which goes to a neighbor E.  A next-hop may provide
   one of the following types of paths:

        Primary Path - This is the primary next-hop.

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

        Unavailable - This may be because the path goes through S to
        reach D, because the link is costed out, etc.


3. Using an Alternate

   If an alternate next-hop is available, the router SHOULD redirect
   traffic to the alternate next-hop when the primary next-hop has
   failed.

   When a local interface failure is detected, traffic that was destined
   to go out the failed interface must be redirected to the appropriate
   alternate next-hops.  Other failure detection mechanisms which detect
   the loss of a link or a node may also be used to trigger redirection
   of traffic to the appropriate alternate next-hops.  The mechanisms
   available for failure detection are discussed in [FRAMEWORK] and are
   outside the scope of this specification.

   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.

3.1 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 remvoed and the network



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   converges according to the deployed routing protocol.

   It is desirable to avoid micro-forwarding loops involving S.  An
   example illustrating the problem is given in Figure 4.  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 4: 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 changed.




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   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
   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.


4. Requirements on LDP Mode

   Since LDP 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.

5. Routing Aspects




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   An SPF-like computation is run for each topology, which corresponds
   to a particular OSPF area or ISIS level.  The IGP needs to determine
   the inheritance of loop-free alternates, as determined for singly
   advertised routes, to multiply advertised routes, for protocols such
   as BGP and LDP and for inter-area or inter-level routes.  These
   alternates are provided to LDP and BGP for forwarding purposes only;
   the alternates are not redistributed in any fashion into other
   protocols.

   The alternate next-hop inheritance is described in the context of
   inter-area routes, but applies equally well to BGP routes and to
   routes which are advertised by multiple routers in the IGP area.


5.1 Multiple-Region Routing

   Routes in different regions inherit their primary next-hops from the
   border routers (area border routers (ABRs) or level boundary routers)
   which offer the shortest path to the destination(s) announcing the
   route.  Similarly, routes must inherit their alternate next-hop and
   will do so from the same border routers.  The shortest path to an
   inter-region route may be learned from a single border router.  In
   that case, both the primary and the alternate next-hops can be
   inherited from that border router.  Figure 5 illustrates this case
   where D is reached via ABR1; the primary next-hop for ABR1 is E and
   the loop-free node-protecting alternate is A1.

                            .............
                      ......             ......
                   ...                         ...
                 ..                               ..
               ..   10  +-----+    5    +-----+  5  ..
              .  +------| A1  +---------| R1  |-----+ .
            ..   |      +-----+         +-----+     | .
            .    |                                +-----+  10
           .     |                 +--------------| ABR1|---------+
           .     |                 |      5       +-----+         |
          .  +-----+     5     +---+-+                .           |
          .  |  S  |-----------|  E  |------------+   .         +-----+
           . +-----+           +-----+   10       |   .         |  D  |
           .     |                                |   .         +-----+
            .    |                                |  .             |
            ..   |     +-----+                  +-----+  20        |
              .  +-----| A2  |------------------| ABR2|------------+
               .   10  +-----+    5             +-----+
                ...                               ...
                   ...                         ...
                      .........................



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            Figure 5: Inter-Region Destination via One Border Router

   The shortest path to an inter-region route may be learned from
   multiple border routers with at least 2 different primary neighbors,
   as is illustrated in Figure 6.  D is reached via ABR1 and ABR2 with
   equal cost from S.  The primary neighbor to reach ABR1 is E1 and the
   alternate is A1.  The primary neighbor to reach ABR2 is E2 and the
   alternate is A2.  In this case, there are equal-cost primary next-
   hops to reach D and they can protect each other.  In this example,
   the primary next-hops would be to E1 and E2; if the link to E2
   failed, then E1 could be used as an alternate and vice-versa.  Thus
   the alternates can be obtained from the primary next-hops.

                            ..........
                      ......          ......
                   ...                      ...
                 ..                            ..
               ..   10  +-----+    5    +-----+  ..
              .  +------| A1  +---------| R1  |-----+
            ..   |      +-----+         +-----+     |.
            .    |             +-----+            +-----+  10
           .     | +-----------| E1  |------------| ABR1|---------+
           .     | |       5   +-----+    5       +-----+         |
          .  +-----+                                   .          |
          .  |  S  |---+  5    +-----+   10            .        +-----+
           . +-----+   +-------| E2  |------------+   .         |  D  |
           .     |             +-----+            |   .         +-----+
            .    |                                |  .             |
            ..   |     +-----+                  +-----+  20        |
              .  +-----| A2  |------------------| ABR2|------------+
               .   10  +-----+    5             +-----+
                ...                            ...
                   ...                      ...
                      ......          ......
                            ..........

                    Figure 6: Inter-Region Destination via
             Multiple Border Routers and Multiple Primary Neighbors


        In the third case, the shortest path to an inter-region route
        may be learned from multiple border routers but with a single
        primary neighbor.  This is shown in Figure 7, where D can be
        equally reached from S via ABR1 and ABR2.  The alternate next-
        hop to reach ABR1 is A1 while the alternate to reach ABR2 is A2.
        It is necessary to select one of the alternates to be inherited.



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                            .............
                      ......             ......
                   ...                         ...
                 ..                               ..
               ..    5  +-----+   15    +-----+ 20  ..
              .  +------| A1  +---------| R1  |-----+ .
            ..   |      +-----+         +-----+     | .
            .    |                                +-----+  10
           .     |                 +--------------| ABR1|---------+
           .     |                 |      15      +-----+         |
          .  +-----+     5     +---+-+                .           |
          .  |  S  |-----------|  E  |------------+   .         +-----+
           . +-----+           +-----+    5       |   .         |  D  |
           .     |                                |   .         +-----+
            .    |                                |  .             |
            ..   |     +-----+                  +-----+  20        |
              .  +-----| A2  |------------------| ABR2|------------+
               .   10  +-----+   15             +-----+
                ...                               ...
                   ...                         ...
                      ......             ......
                            .............

                    Figure 7: Inter-Region Destination via
               Multiple Border Routers but One Primary Neighbor


5.1.1 Inheriting Alternate Next-Hops with One Primary Neighbor

   The main question when deciding whether an alternate can be inherited
   is whether or not that alternate will continue to provide the
   necessary protection.  I.e., will the alternate continue to be usable
   as an alternate and provide the same link or node protection with
   respect to the destination that was provided with respect to the
   border router.  It can be proved that the alternate will be usable as
   an alternate and provide at least the same link or node protection
   that was provided with respect to the border router.  The alternate
   next-hop inheritance procedure SHOULD select a loop-free node-
   protecting alternate, if one is available.

5.1.2 OSPF Inter-Area Routes

   In OSPF, each area's links are summarized into a summary LSA, which
   is announced into an area by an Area Border Router.  ABRs announce
   summary LSAs into the backbone area and inject summary LSAs of the
   backbone area into other non-backbone areas.  A route can be learned
   via summary LSA from one or more ABRs; such a route will be referred
   to as a summary route.



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   The alternate next-hop inheritance for summary routes is as described
   in Section 5.1.1


5.1.3 OSPF External Routing

   Rules of inheritance of alternate next-hops for external routes is
   the same as for inter-area destinations.  The additional complication
   comes from forwarding addresses, where an ASBR uses a forwarding
   address to indicate to all routers in the Autonomous System to use
   the specified address instead of going through the ASBR.  When a
   forwarding address has been indicated, all routers in the topology
   calculate the shortest path to the link specified in the external
   LSA.  In this case, the alternate next-hop of the forwarding link
   should be used, in conjunction with the primary next-hop of the
   forwarding link, instead of those associated with the ASBR.


5.1.4 ISIS Multi-Level Routing

   ISIS maintains separate databases for each level with which it is
   dealing.  Nodes in one level do not have any information about state
   of nodes and edges of the other level. ISIS level boundary points ,
   also known as ISIS level boundary routers, are attached to both
   levels.  ISIS level boundary routers summarize the destinations in
   each, level. ISIS inter-level route computation is very similar to
   OSPF inter area routing.  Rules for alternate next-hop inheritance is
   the same as described in Section 5.1.1


5.2 OSPF Virtual Links

   OSPF virtual links are used to connect two disjoint backbone areas
   using a transit area.  A virtual link is configured at the border
   routers of the disjoint area.  There are two scenarios, depending
   upon the position of the root, router S.

   If router S is itself an ABR or one of the endpoints of the disjoint
   area, then router S must resolve its paths to the destination on the
   other side of the disjoint area by using the summary links in the
   transit area and using the closest ABR summarizing them into the
   transit area.  This means that the data path may diverge from the
   virtual neighbor's control path.  An ABR's primary and alternate
   next-hops are calculated by S on the transit area.

   The primary next-hops to use are determined based upon the closest
   set of equidistant ABRs; the same rules described in Section 5.1.1
   for inter-area destinations must be followed for OSPF virtual links



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   to determine the alternate next-hop.  The same ECMP cases apply.

   If router S is not an ABR, then all the destinations on the other
   side of the disjoint area will inherit the virtual link's endpoint,
   the transit ABR.  The same OSPF inter-area rules described in Section
   5.1.1 must be followed here as well.

   A virtual link cannot be used as an alternate next-hop.


5.3 BGP Next-Hop Synchronization

   Typically BGP prefixes are advertised with AS exit routers router-id,
   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 the all the IGP destinations, which
   includes 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 continue to use the IGP optimal distance to
   find the nearest exit router.  MBGP routes do not need to copy the
   alternate next hops.


5.4 Multicast Considerations

   Multicast traffic is out of scope for this specification of IP Fast-
   Reroute.  The alternate next-hops SHOULD not used for multi-cast RPF
   checks.


6. Security Considerations

   This document does not introduce any new security issues. The
   mechanisms described in this document depend upon the network
   topology distributed via an IGP, such as OSPF or ISIS.  It is
   dependent upon the security associated with those protocols.


7.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights."

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET



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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


8. References

   [FRAMEWORK] M. Shand, "IP Fast Reroute Framework", draft-ietf-rtgwg-
   ipfrr-framework-01.txt, June 2004

   [LDP] L. Anderson, P. Doolan, N. Feldman, A. Fredette, B. Thomas,
   "LDP Specification", RFC 3036, January 2001

   [RSVP-TE] D. Awduche, L. Berger, D. Gan, T. Li, V Srinivasan, G.
   Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209,
   December 2001

   [RSVP-TE FRR] P. Pan, D. Gan, G. Swallow, JP Vasseur, D. Cooper, A.
   Atlas, and M. Jork, "Fast Reroute Extensions to RSVP-TE for LSP
   Tunnels", work-in-progress draft-ietf-mpls-rsvp-lsp-fastreroute-
   07.txt, June 2004

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

   [RFC3277] D. McPherson, "Intermediate System to Intermediate System
   (IS-IS) Transient Blackhole Avoidance", RFC 3277, April 2002

   [ISIS] R. Callon, "Use of OSI IS-IS for Routing in TCP/IP and Dual
   Environments", RFC 1195, December 1990

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

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

   [RFC2370] R. Coltun, "The OSPF Opaque LSA Option", RFC 2370, July
   1998

9. Authors Information

   Raveendra Torvi
   Avici Systems
   101 Billerica Avenue
   N. Billerica, MA 01862
   USA
   email: rtorvi@avici.com



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   phone: +1 978 964 2026

   Gagan Choudhury
   AT&T
   Room D5-3C21
   200 Laurel Avenue
   Middletown, NJ 07748
   USA
   email: gchoudhury@att.com
   phone: +1 732 420-3721

   Christian Martin
   Verizon
   1880 Campus Commons Drive
   Reston, VA 20191
   email: cmartin@verizon.com

   Brent Imhoff
   WilTel Communications
   3180 Rider Trail South
   Bridgeton, MO 63045
   USA
   email: brent.imhoff@wcg.com
   phone: +1 314 595 6853

   Don Fedyk
   Nortel Networks
   600 Technology Park
   Billerica, MA 01821
   email: dwfedyk@nortelnetworks.com
   phone: +1 978 288 3041

10. Editor's Information


   Alia Atlas
   Avici Systems
   101 Billerica Avenue
   N. Billerica, MA 01862
   USA
   email: aatlas@avici.com
   phone: +1 978 964 2070









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