In today’s post, we will talk about one very cool feature in OSPF.   As you know, OSPF uses the Dijkstra’s algorithm to calculate the shortest path tree or SPT and create a topology graph.   This topology is then used to populate the routing table.

Now, when a change occurs in the network, link-state advertisements (LSAs) are flooded to inform all members of a given area about the changes.   When a change to a Type-1 or Type-2 LSAs occurs in an area, the entire SPT has to be recalculated. This is the normal behavior of OSPF.  Now, the problem arises in cases where the change affects only to a small section of the tree.    In this particular scenario, there is no need to recalculate the entire SPT because most of the SPT remain unchanged.    Here is where ISPF enters to play.

Incremental SPF allows the recalculation of the affected area without the overhead caused from full SPF recalculation as long as the affected link is a leaf in the SPT.   The result is faster convergence and the reduction of CPU utilization.

Configuring ISPF

To configure ISPF use the ispf OSPF process command.

Worth mentioning that routers with ISPF configured can operate with routers without this feature enabled.

It is time to close this post.  In the next one, we will discuss some additional OSPF features.

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