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Fast Convergence: Partial SPF calculation

Shortest path first (SPF) is the algorithm used by IS-IS and OSPF routing protocols to calculate the topological information from the received link state updates. You can find more information about SPF calculation follow the link Dijkstra’s algorithm.

Partial SPF is an efficient shortcut used by the routers to speed up the process of route calculations and hence achieve faster convergence. This feature is implemented by default mostly in all known SPF implementations.

Instead of running the Full SPF run for every prefix or metric change in the network, the routers will run only Partial SPF calculation by only scanning the prefixes information to extract the changed information. This eliminates the need for recalculating the full topology information  (Full SPF Run).

Partial Route calculation (PRC) is usually triggered by one of the following reasons:

  • Metric of Prefix change.
  • Adding or deleting prefixes.

PRC is implemented by OSPF and IS-IS for both IPv4 and IPv6. IS-IS implementation is more efficient than OSPF due to the way IS-IS handles the prefix information. In IS-IS PRC can be used for all leaf information changes while OSPFv2 supports PRC for LSA type 3,4,5,6,7 only but not LSA type 1 or 2. Sometimes using redistribution in OSPFv2 is not a bad idea and is the right way to scale your single area network :) .

I have tried to make things here as simple as possible, but for those who seek more information will need to get in depth understanding of the SPF algorithm operations.

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