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Understanding IS-IS Areas to Divide an Autonomous System into Smaller Groups

In IS-IS, a single AS can be divided into smaller groups called areas.

Link-state protocols cannot scale well if a large autonomous system (AS) consists of a single set of routing devices that all share a common database to compute the best paths through the AS. Because the shortest-path-first (SPF) algorithm works in an exponential fashion, the CPU demand can become too heavy when too many routing devices share their complete routing information with each other. To alleviate this issue, large ASs are divided into smaller parts called areas.

When ASs are split into areas, the disjointed areas must be connected to route traffic between the areas. Reachability information at the area borders must be injected into each other areas.

In IS-IS, routing between areas is organized hierarchically. This organization is accomplished by configuring Level 1 and Level 2 intermediate systems. Level 1 systems route within an area. When the destination is outside an area, Level 1 systems route toward a Level 2 system. Level 2 intermediate systems route between areas and toward other ASs. No IS-IS area functions strictly as a backbone.

Level 1 routers share intra-area routing information, and Level 2 routers share interarea information about IP addresses available within each area. Uniquely, IS-IS routers can act as both Level 1 and Level 2 routers, sharing intra-area routes with other Level 1 routers and interarea routes with other Level 2 routers.

The propagation of link-state updates is determined by the level boundaries. All routers within a level maintain a complete link-state database of all other routers in the same level. Each router then uses the Dijkstra algorithm to determine the shortest path from the local router to other routers in the link-state database.