LFA Tiebreakers (OSPF)
Load basic.ospf.ldp.enabled.cfg
Configure the R6-R7 link to have a metric of 25.
Configure LFA on R3 for the Gi0/0/0/5 link. Prefer node-protecting LFAs over SRLG-disjoint LFAs, and SRLG-disjoint LFAs over lowest-backup-metric paths.
Answer
Explanation
Before changing the tiebreakers, let’s look at the default settings.
First, notice that this output looks different than ISIS, because there is only one section with tiebreakers. In OSPF, LFA and TI-LFA both use the same tiebreakers, unlike ISIS.
Interestingly, all the indexes 10-40 match ISIS, however OSPF treats index numbers differently! The highest index value is best. Due to this, we should see the higher metric path via R6 wins because it is node protecting, while the lower metric path via R9 is not:
The “No tunnel” rule means that OSPF will give preference to a path that does not use a tunnel interface. The “interface disjoint” rule seems unnecessary, because a backup path must by definition use a different interface. Both of these rules cannot be preempted, because the highest manual index value we can configure is 255. Perhaps these two rules are needed to make IPFRR work within OSPF for some reason.
Let’s now add in our custom tiebreakers. Unlike ISIS, the tiebreakers will be merged with the default values.
This has not changed our backup path selection, because node protection was always the next-highest tiebreaker. However, we can verify that it wins over SRLG disjoint by configuring Gi0/0/0/5 and Gi0/0/0/6 in the same SRLG. R6 should still be used as the repair path.
The repair path is no longer SRLG-disjoint:
If we move the srlg-disjoint tiebreaker above node protection, we will now prefer R9:
The rest of the tiebreaker rules work in the same way as we saw with ISIS. You just need to remember that with OSPF, highest index values are best.
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