Hey,
There seem to be some conflicting suggestions for ISIS fast convergence
timers, and I can’t seem to understand why that would be. The former example
is ISIS in a LFA FRR environment, the latter is from a general best practise
guide. I can’t imagine LFA FRR or not would matter to the best practise would
it?
ASR920 + LFA FRR[1]:
spf-interval 5 50 200
prc-interval 5 50 200
lsp-gen-interval 5 50 200
IE: XE 16.5 (Everest) (which can also run on an ASR920)[2]:
spf-interval 5 1 50
prc-interval 5 1 50
lsp-gen-interval 5 1 50
SPF timers is generally a design decision, so the values above are just
reflecting different design approaches: Choosing an initial wait of 1ms (the
latter settings, i.e. spf-interval 5 1 50) tunes the network for optimal
reaction for link failures, so routers will immediately start to re-route, with
the risk of a subsequent SPF required if the failure was a node failure and
additional LSP updates from other nodes are required to judge this.
Hence a little more conservative setting uses 50msec initial wait, allowing
more LSP updates to come in before the new SPF is calculated..
With LFA-FRR, core failures can be handled differently, so a little less
aggressive initial-wait makes a lot of sense in this case..
Either way: even without LFA-FRR, difference between 1 and 50 msec is marginal
and not noticeable in practice, so why bother much __
oli
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