Fast Convergence: IS-IS performance tuning

IS-IS is the most selected protocol by service providers and large carriers all over the world; this is what makes the understanding  of this protocol important. We have been discussing fast convergence and high availability in the latest few posts and this post is not different, I am just going to give you an overview on the timers available for tuning in IS-IS. In later posts we will be dicussing the best practices in deploying this protocol and tuning for fast convergence.

All vendor implementation have the following timers available, but may be in different defaults and some of them can be modified and some can not be. You will have to check your vendor's documentation for specific guide lines.

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 trigers 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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