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LFAs and OSPF with multiple intra-area AS-external or summary-routes

Subject: LFAs and OSPF with multiple intra-area AS-external or summary-routes
From: Alia Atlas
Date: Mon, 07 Feb 2005 20:56:20 -0500
The behavior of loop-free alternates within a single area seems pretty clear. Things get a bit more interesting when considering multiple areas for OSPF. For instance, consider the following topology where the --- lines indicate area 1 and the **** lines indicate area 2.
                    [ A2 ] ************** [ Z ]
                      *                     *
                      *                     *
                   10 *                     *
             10       *    1                *
  [ A1 ] ---------- [ S ]*****[ N2 ]        *
    |                 |         *           *
  5 |               5 |       1 *           *
    |       5         |    1    *    13     *
  [ X ] ----------- [ N1 ]----[ Y ] **** [ ASBR2]
    |                                       *
  5 |                                       * 10
    |        10                             *
  [ ASBR 1] ---- P                          P

In the above topology, S learns two routes to the prefix P. From area 1, S learns an intra-area route to ASBR1 with a cost of 15 (25 to P); similarly from area 2, S learns an intra-area route to ASBR2 with a cost of 15 (25 to P).
Assuming that S is doing ECMP, S would normally select both N1 and N2 as
primary next-hops. The question is what should S select for alternates. S
could assume that N1 and N2 can be alternates for each other & provide both
link and node protection. However, Y is also an ABR & sees a shorter path
to the prefix P via N1 in area 1. Thus, although the primary path computed
by S for area 2 didn't use N1, traffic forwarded to N2 will go to Y and
thus to N1.
Therefore, the two primary next-hops can't be guaranteed to provide
node-protection. Similarly, there could be a broadcast interface between
S, N1 and Y in the above topology - and then link protection wouldn't be
In a more absurd topology, S could not even have the topology information
available to in any way determine this. This would be the case when 2 ABRs
were crossed.
This seems to me to indicate that a primary next-hop SHOULD be protected by
an alternate next-hop from the same area; this implies the need to store an
alternate per (non-backbone) area primary next-hop to gain protection.
Also, the ability of an alternate path to exit the area and (potentially)
return via the "protected" primary neighbor or a link to that neighbor may
need to be considered. This could happen by crossing 2 ABRs, one of which
had a shorter intra-area route in the second area & the other which had a
shorter intra-area route in the first area - that was back through the
"protected" primary neighbor.
I believe that both of the above are potentially of concern for inter-area
routes and AS-external routes.
The first seems like it could be solved by requiring a primary next-hop be
protected by an alternate from the same area - even if there is an
equal-cost path via a different area. The second seems more theoretically
problematic - though less realistic. These issues appear to only exist for
the ABRs.
Thoughts? Have I missed anything? Any suggestions on reasonable
compromise approaches?

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