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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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.