IS-IS Route Summarization

Route summarization provides many advantages, which includes reducing the router resources (CPU and memory) required due to reduced routing table size, and scoping or isolating route flapping issues within an area. When a router is unaware of a change or problem in the network, its LSDB is not being updated, and SPF recalculation is not being performed. Summarization allows routers within areas to only maintain and manage internal knowledge of the areas they reside and summarize the knowledge across area boundaries.

IS-IS allows manual summarization with some limitations. IS-IS internal route summarization within an area is not viable; therefore it cannot be implemented and configured on an L1 router. Multiple L1 internal routes can be summarized between areas as a single L2 route. An L1/L2 router (area border router) is where route summarization is being implemented.

IS-IS route summarization requires extra attention when there are multiple L1/L2 routers between 2 areas. If one of them is summarizing routes, others must perform summarization too. As if one of them is advertising a summary route while others advertise the more specific routes, packets will be routed through other routers due to the most-specific prefix matching rule.

The rules for route summarization on an OSPF ABR are applicable to Integrated IS-IS.
Below lists the rules for summarizing IP routes in Integrated IS-IS:

• L1 routes cannot be summarized within an area. ^[1]
• L1/L2 routers can summarize the internal routes within their areas. The summarized routes are propagated into the L2 backbone; similar to summarization on an OSPF ABR.
• If route summarization is configured on an L1/L2 router, the same configuration must also be implemented on all L1/L2 routers reside in the area to prevent suboptimal routing.

[1] Cisco IOS allows the configuration of the summary-address IS-IS router subcommand on an L1 router without any warning message; though it has no effect on an L1 router.

The summary-address {prefix} {mask} [level-1 | level-1-2 | level-2] [metric metric] IS-IS router subcommand creates an L1 or L2 aggregate or summarized address. External routes learned from other routing protocols can also be summarized.

level-1	Only routes redistributed into L1 are summarized with the configured prefix mask.
level-1-2	Summary routes are applied when redistributing routes into L1 and L2 IS-IS, and when L2 IS-IS advertises L1 routes as reachable in its area.
level-2	Routes learned by L1 routing are summarized into the L2 backbone with the configured prefix mask. Routes redistributed into L2 IS-IS will be summarized too.

The following line will be seen on an L1/L2 summarizing router.
i su prefix/mask [115/10] via 0.0.0.0, Null0

The i su code indicates an IS-IS summary route; while the Null route is inserted into the routing table automatically upon configuring an IS-IS summary route to prevent routing loops from occurring in case receiving packets that match the summary route but the summarizing router does not have a more-specific route.

The drawback of route summarization is that routers might have lesser information to calculate the most optimal paths for all destinations.