Cisco Systems 78-11741-02 Partial Route Computation PRC, Sets the LSP refresh interval, IPC-287

Configuring Integrated IS-IS

Miscellaneous IS-IS Parameters Configuration Task List

To change the LSP refresh interval or lifetime, use the appropriate command in router configuration mode:

Command		Purpose
Router	(config-router)#lsp-refresh-interval	Sets the LSP refresh interval.
seconds
Router	(config-router)# max-lsp-lifetime	Sets the maximum time that link-state packets (LSPs) can remain in
seconds		a router’s database without being refreshed.

Customizing IS-IS Throttling of LSP Generation, SPF Calculation, and PRC

Partial Route Computation (PRC)

PRC is the software’s process of calculating routes without performing an SPF calculation. This is possible when the topology of the routing system itself has not changed, but a change is detected in the information announced by a particular IS or when it is necessary to attempt to reinstall such routes in the RIB.

Benefits of Throttling IS-IS LSP Generation, SPF Calculation, and PRC

IS-IS throttles three main events: link-state PDU (LSP) generation, Shortest Part First (SPF) computation, and partial route computation (PRC). Throttling slows down the frequency of these events during times of network instability. Although throttling these events slows down network convergence, not throttling could result in a network not functioning. If network topology is unstable, throttling slows down the scheduling of these intervals until the topology becomes stable.

The throttling of LSP generation prevents flapping links from cause many LSPs to be flooded through the network. The throttling of SPF computation and PRC prevents the router from crashing from the demand of too many calculations.

How Throttling of IS-IS LSP Generation, SPF Calculation, and PRC Works

IS-IS throttling of LSP generation, SPF calculations, and PRC occurs by default. You can customize the throttling of these events with the lsp-gen-interval,spf-interval, and prc-intervalcommands, respectively.

The arguments in each command behave similarly. For each command:

•The first argument indicates the maximum number of seconds between LSP generations or calculations.

•The second argument indicates the initial wait time (in milliseconds) before running the first LSP generation or calculation.

•The third argument indicates the minimum amount of time to wait (in milliseconds) between the first and second LSP generation or calculation. (In addition to this wait time, there might be some other system overhead between LSP generations or calculations.)

Each subsequent wait interval is twice as long as the previous one until the wait interval reaches the maximum wait time specified, upon which the wait interval remains constant. After the network calms down and there are no triggers for 2 times the maximum interval, fast behavior is restored (the initial wait time).

Cisco IOS IP Configuration Guide

IPC-287