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RFC 5286         IP Fast Reroute: Loop-Free Alternates    September 2008


                                 <@@@
                           @@@>
                    +-----+       +-----+
                    |  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-
   looping in the event of node failure.  There are topologies where
   there may be either a node-protecting 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

Atlas, et al.               Standards Track                     [Page 6]