OSPF Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)

Lessons Discussion
3

Hello Luis

The official definition according to RFC 4790 is the following:

  The P-space of a router with respect to a protected link is the
  set of routers reachable from that specific router using the pre-
  convergence shortest paths without any of those paths (including
  equal-cost path splits) transiting that protected link.

  For example, the P-space of S with respect to link S-E is the set
  of routers that S can reach without using the protected link S-E.

So for R1, the routers that are reachable without the use of the failed link are R2 and R3 under normal circumstances . R4 is actually not reachable because it is reached via the link to R5.

The same goes for the Q-space, which includes R4 and R3, but not R2, because R2 would have gone through R1 to get to R5.

Remember that this feature is used particularly for loop topologies. The point of finding the P and Q spaces, is to find the PQ node, which is the only router that exists in both the P and the Q spaces. This is the first router along the loop that will have an OSPF route R5, and this is the router that is identified as the one with which the tunnel must be made.

I hope this has been helpful!

Laz

PS Thanks for pointing out that there was no forum topic for this lesson. It has been fixed…

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