iSPF for OSPF and IS-IS

Terry Slattery
Principal Architect

Pete Welcher at Chesapeake NetCraftsmen ( informed me about the benefits of incremental SPF for the link-state protocols OSPF and IS-IS, having read it in Russ White’s book, Optimal Routing Design, while studying for his CCDE test.  This feature has been around a while, having been added to Cisco IOS 12.0S, but I’ve not seen it used in production networks.  In a way, it is similar to the concept that researchers at UCSF came up with and that I wrote about last week (Update to Link-State Routing Algorithms).  iSPF recalculates only the affected part of the SPF tree as long as the affected link or node is a leaf, significantly reducing the need for many routers within the area from having to recalculate the entire topology. Even better, it is compatible with routers that don’t run iSPF, making a transition easy to perform.

There’s also an IEEE paper on the performance of OSPF and iSPF, Framework for performance engineering of OSPF software, I only read the abstract and it sounds like an interesting paper for the UCSF researchers to review and include in their new algorithm’s analysis.

Adding iSPF to either OSPF or IS-IS is very easy.

router ospf 1

IS-IS can specify level-1, level-2, or level-1-2 (the latter is shown here):

router isis
 ispf level-1-2

Please post a comment if you know of any problems with using it.  Good luck in making your network more efficient.


Re-posted with Permission 

NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under


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