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:
- Level-2 (the backbone)
- Level-1 (the areas)
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: