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Re: LFAs and ECMP

Subject: Re: LFAs and ECMP
From: Alia Atlas
Date: Wed, 16 Feb 2005 06:11:43 -0500
Here is the text that I'm adding to the LFA draft. Please let me know if this covers the issues sufficiently.
2.4  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
   concern.

                           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 L1 to E1 can obtain
   link and node protection from L3 to E3, which is one of the other
   primary next-hops; L1 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 E3 can
   obtain link and node protection from L2 to E1, but can only get link
   protection 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
   cases.


Alia


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