OSPF’s Maximum Metric

There are some situations where you just don’t want any traffic flowing through a particular router unless there are ABSOLUTELY NO other alternatives. How can we do that? Tell a particular router to advertise his/her LSAs with the maximum metric possible (0xFFFF).

There are several good reasons to make use of this feature listed here. I’m particularly fond of the third option, gracefully removing a router. This could also apply to maintenance windows. If I know I’m going to be knocking out a transit link, I could force neighboring routers to recalculate a next-hop before I knock out the link. This way no packets are black-holed while SPF calculations are performed.

Assume I need to get from R0 to R3 (1.2.3.4) and my only transit routers are R1 and R2. R0 is currently using CEF to load-balance over the two paths:

If I were to unplug R1 without warning, a packet or two might be on its way from R0 to R1 to R3, only to die at R1 due to a power loss. Instead, I’ll tell R1 to advertise the maximum possible metric:

Now on R0 there’s only one route to 1.2.3.4 because the metric in R1’s LSA is 65535:

This will be handy.

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One comment

  • Where are the updates to this site? I miss the dry wit and precise, quantitative solutions to inefficiency that it offered.

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