Defining the SPF Route Calculation Level

The IS-IS protocol uses the Dijkstra algorithm to compute IP node metrics when a change occurs within the IS-IS network. This calculation results in the IS-IS router containing a shortest-path tree (SPT) that maps the shortest path to each node in the IS-IS network.

By default, the router uses a partial route calculation (PRC) SPF to determine the next hop (when required). This partial computation occurs when the router receives link-state PDUs (LSPs) with only changes relating to IP prefixes (for example, the addition of a new IP prefix, change in attributes of an existing IP prefix, or the removal of an existing IP prefix).

Because changes in IP prefixes happen more frequently than other events, using the PRC SPF results in faster IS-IS convergence and saves router resources. However, you can also specify that the router always use full SPF, recalculating the entire SPT, when resolving any IS-IS state changes.

full-spf-always

• Use to enable and disable full SPF calculations for IS-IS network changes.
• Example

  host1(config-router)#full-spf-always

• Use the no version to restore partial route calculation (PRC) mode for SPF calculations.
• See full-spf-always

Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice.