Remote LFA or TILFA?
Last updated
Last updated
Load rlfa.or.tilfa.init.cfg
The R4-R10 link metric is now 20. Is RLFA possible, or is TILFA required? Test this out by configuring the protection you believe is required.
The answer is that RLFA is possible.
You might think there is currently no PQ node, which would require TILFA. R3 can only reach R5 and R6 without traversing the protected link. R3 cannot reach R10 without traversing the link, as it has ECMP paths to R10.
However, R3 can use an “Extended P” space. Because R3 simply switches a packet out a given interface, R3 can use the P space of its own neighbors and extend its P-space. So R3 can use R5’s P space. This is because when R5 receives a packet destined for R10, R5 does not use the protected link. (This is called the extended P space because you are now looking at R5’s P space, not R3’s P space).
The diagram now looks like this:
R10 is available as a PQ node. The label stack is as follows: <R5’s label for R10>/<R10’s label for 4.4.4.1>.
We can verify this using the following show commands:
24005 is R5’s LDP label for 10.10.10.1/32. 24002 is R10’s label for 4.4.4.1/32.
No matter what the metric of the R4-R10 link is, R5 can always reach it without crossing the R3-R4 link (as long as no other link metrics change). So the metric at which R10 no longer becomes a Q node is when RLFA is no longer possible. (Put another way, an RLFA is no longer possible when R10 does not use the R10-R4 link as the only path to get to 4.4.4.1/32).
The metric via the path R10-R6-R5-R3-R4 is 40. If the link metric becomes at least 40, we no longer have a PQ node.
R3 no longer has a backup path for 4.4.4.1/32:
R10 now has ECMP paths to 4.4.4.1, so it is no longer a Q node. It uses the protected link to reach 4.4.4.1/32.
