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


   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
   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 Non-Broadcast Multi-Access (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 that 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

Atlas, et al.               Standards Track                    [Page 11]