IS-IS and Fast Convergence

by Wael Osama

Wael Osama Bio: Wael Osama is a senior network engineer, working in one of the largest service providers in Egypt. His main focus is service providers backbone technologies including IGPs, BGP and MPLS. More information about Wael can be found on his linkedin profile http://www.linkedin.com/in/wosama. Wael’s blog has posted an interview of me. http://networkers-online.com/blog/?p=170 You can find more articles from Wael at http://networkers-online.com/blog/.

IS-IS is the most selected IGP by service providers and large carriers all over the world. This makes the deep understanding of this protocol an important skill. In this article I am just going to give you an overview about the timers available in IS-IS that can be tuned for fast convergence. In later articles we may discuss the best practices of deploying IS-IS and tuning it for fast convergence.

All vendors’ implementations have the listed below IS-IS timers available. You have to check your vendor’s documentation for specific guide lines on modifying these timers.

Neighbor detection timers:
• Hello Interval: this is the time between two consequent hello packets. The timer determines how fast a neighbor can be discovered. Default is 10 seconds for all interfaces and 3 seconds for DIS interfaces.
• Hold down time: this is the time that the router can wait without seeing a hello from the neighbor before declaring the relationship down. In IS-IS you can set the hold down timer using the hello-multiplier command which is 3 by default. Also you can set the timer to 1 second using the minimum key word in the hello-interval command.

BFD is a very good alternative for modifying the above timers for fast detection in most cases.

Link State packets timers:
• LSP refresh interval: This is the time the router will wait before re-flooding its own generated LSPs. The default timer is 15 min and the maximum time is about 18 hours.
• LSP lifetime: This is the interval in which the LSP can reside in the link state database before being purged. This timer must be set to be more than the LSP refresh interval.
• LSP generation interval: Specifies the number of seconds between creating new versions of a given LSP on a per node basis. SPF calculation timers:
• PRC interval: Specifies the time between two consecutive PRCs. Find information here about what triggers the PRC calculation.
• SPF Interval: Specifies the number of seconds between two consecutive SPF calculations.

There are no best practice specific values for these timers; there are only  best practice guide lines to set your timers. The timers may vary from network to network based on the selected vendors, platforms used, interoperability and the network size and design.

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