The RIB processing of specific routes can be prioritized through the use of the rib-priority command. This command allows specific routes to be prioritized through the protocol processing so that updates are propagated to the FIB as quickly as possible.
The rib-priority command is configured within the global IS-IS routing context, and the administrator has the option to either specify a prefix-list or an IS-IS tag value. If a prefix list is specified then route prefixes matching any of the prefix list criteria is considered high priority. If instead an IS-IS tag value is specified then any IS-IS route with that tag value is considered high priority.
The routes that have been designated as high priority are the first routes processed and then passed to the FIB update process so that the forwarding engine can be updated. All known high priority routes should be processed before the IS-IS routing protocol moves on to other standard priority routes. This feature has the most impact when there are a large number of routes being learned through the IS-IS routing protocols.