IS-IS Levels & Relationships

IS-IS creates flooding boundaries logical by breaking the network into routing domain levels i.e. A level2 routing domain and multiple level1 routing domains. The single Level 2 routing domain could be analogous to OSPF Area0 and multiple Level 1 routing domains are analogous to other OSPF Areas. Basically, IS-IS has two layer hierarchy:

  1. Level-2 (the backbone)
  2. Level-1 (the areas)
This two layer hierarchy is achieved by dividing the routers (IS’s) into three types called L-1, L-2, L-1-2 IS’s.
Level1 Router is analogous to OSPF Internal non-backbone router (Totally Stubby). It contains a level1 LSDB only and is responsible for only routing to ESs inside an area. Level1 routers maintain the Level1 database for the area and exit points to neighboring areas.
Level2 Routers on the other hand are analogous to OSPF Internal Backbone router. They contain a level2 LSDB only. They are responsible for routing between areas and they Interconnects the Level 1 areas. L2 Routers store separate database of only inter-area topology & have all interfaces in the same area.
level-1-2 routers contain two separate LSDB’s for level-1 & Level-2 and are analogous to OSPF ABR router. They participate in both L1 intra-area routing and L2 inter-area routing. Main function of L1-2 routers is to support Level1 function communicating with other Level1 routers in their area, inform other Level 1 routers that they are the exit point (default route) from the area. and support Level 2 function communicating with the rest of the backbone path.

By default, IS-IS enables both Level 1 and Level 2 operations on IS-IS routers (in Cisco & most of the vendors). If a router is to operate only as an area router or only as a backbone router, this needs explicit configuration.

This two-level hierarchy of IS-IS is supportive for large routing domains. A large domain may be administratively divided into areas. Each system resides in exactly one area. Routing within an area is referred to as Level 1 routing. Routing between areas is referred to as Level 2 routing. A Level 2 Intermediate System (IS) keeps track of the paths to destination areas. A Level 1 IS keeps track of the routing within its own area. For a packet destined for another area, a Level 1 IS sends the packet to the nearest Level 2 IS in its own area, regardless of what the destination area is. Then the packet travels via Level 2 routing to the destination area, where it may travel via Level 1 routing to the destination. It should be noted that selecting an exit from an area based on Level 1 routing to the closest Level 2 IS might result in suboptimal routing.

There are some hard rules for IS-IS routers to communicate with each other & form adjacency. Please review my IS-IS Post for further detail:

Adjacencies in IS-IS


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Niam Fress

Which parts support authentication in IS-IS?


Thanks for the post. It is helping me for my masters




Good Post!!