[c-nsp] ISIS Fast Convergence (ASR920?)

[Oliver Boehmer (oboehmer)]

    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