You can tag IS-IS routes to configure route leaking (redistribution). Since only the appropriate routes are redistributed--or leaked--the results is network scalability and faster convergence for the router update. If you configure route leaking and you want to match on a tag, use a route map (not a distribute list).
There are two general steps to using IS-IS route tags: tagging routes and referencing the tag to set values for the routes and/or redistribute routes.
There are three ways to tag IS-IS routes: tag routes for networks directly connected to an interface, set a tag in a route map, or tag a summary route. All three methods are described in this section. The tagging method is independent of how you use the tag.
After you tag the routes, you can use the tag to set values (such as a metric, or next hop, and so on) and/or redistribute routes. You might tag routes on one router, but reference the tag on other routers, depending on what you want to achieve. For example, you could tag the interface on Router A with a tag, match the tag on Router B to set values, and redistribute routes on Router C based on values using a route map.
If someone mistakenly injects a large number of IP routes into IS-IS, perhaps by redistributing Border Gateway Protocol (BGP) into IS-IS, the network can be severely flooded. Limiting the number of redistributed routes prevents this potential problem. You can either configure IS-IS to stop allowing routes to be redistributed once your maximum configured value has been reached or configure the software to generate a system warning once the number of redistributed prefixes has reached the maximum value. Before configuring the tasks in this section, you should be familiar with the following concept.
LSP Full State
In some cases when a limit is not placed on the number of redistributed routes, the LSP may become full and routes may be dropped. A user can specify which routes should be suppressed in that event so that the consequence of an LSP full state is handled in a graceful and predictable manner.
Redistribution is usually the cause of the LSP full state. By default, external routes redistributed into IS-IS are suppressed if the LSP full state occurs. IS-IS can have 255 fragments for an LSP in a level. When there is no space left in any of the fragments, an LSPFULL error message is generated.
Once the problem that caused the LSP full state is resolved, a user can clear the LSPFULL state.
In order to speed up IS-IS convergence, the number of IP prefixes carried in LSPs needs to be limited. Configuring interfaces as unnumbered would limit the prefixes. However, for network management reasons, you might want to have numbered interfaces and also want to prevent advertising interface addresses into IS-IS. There are two alternative methods to avoid the overpopulation of routing tables and thereby reduce IS-IS convergence time. In order to choose the method that will work best for your network, you should become familiar with the following concepts.