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


   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 subset of loop-free alternates are downstream paths that 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, 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 links.

   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
   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.


Atlas, et al.               Standards Track                     [Page 5]