LSP Flooding and SPF Recalculation Timers
IS-IS, like OSPF, uses a complex, recursive algorithm for calculating the best path to a particular destination and ages out LSPs every so often. The intervals where these events normally occur are configurable on Cisco routers. Chapter 6, "IS-IS Network Design," has some information on the importance of adjusting these timers in large IS-IS networks.
To adjust the interval at which IS-IS does a SPF run, use the spf-interval command. The default interval is 5 seconds. IS-IS will automatically run SPF each time a change in the network occurs, regardless of whether this interval of time has passed.
Each LSP advertised also contains a Remaining Lifetime field (also known as the max-lsp-lifetime or Maxage) ...
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