Tuesday, 9 December 2008

Commands: ispf

Looking through OSPF I came across a command I did not know about before. The command I came across was ispf, which stands for incremental shortest path first. The benefits of ispf can best be summed up from Cisco's own page on the topic.

OSPF uses Dijkstra's SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the calculating router itself, then the full SPT is performed.

Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.

Looks like quite a useful command to remember, so I will be adding it to my configs in future.

2 comments:

Calin said...

Recently I came over a config that had issued under "router ospf" this ispf and I had not idea what it is, and still I'm not working with OSPF since yesterday :). From the description seems very useful, but I have not idea what impact will have on a real environment network. Maybe I will test it and let you know how it is working.

Luis Garcia said...

use it on the 6500's along with the nsf command. We use it on all our 6500's. Basically takes advantage of redundant supervisor engine cards.