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SPF events in OSPF and IS-IS

The Shortest Path First (SPF) algorithm used by all (read: both) popular link-state routing protocols should be well-known to all network engineers, but the fact that the IP routing table gets populated by a distance-vector-like second phase of the algorithm is oft forgotten.

Check this short presentation in case you’d like to get a bit more in-depth explanation of the SPF algorithm.

The second phase of the link state route selection algorithm is called Partial SPF in OSPF and Partial Route Calculation (PRC) in IS-IS. Obviously it’s beneficial if the router can react to a change in the network topology by running PRC, not the full SPF, and there are significant differences in the way OSPF and IS-IS respond to various topology change events. The differences are not architectural; you could make OSPF behave as well as IS-IS. They just prove that the old wisdom is still true: very large IOS-based networks use IS-IS because it’s better implemented than OSPF.

Read the SPF events in OSPF and IS-IS article in the CT3 wiki

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