In IS-IS, routers are classified into the following types: Level -1, Level-2, or Level-1-2
Level-1: A Level-1 router is an internal router in an IS-IS area. A Level-1 router can establish neighbor relationships only with other Level-1 routers in the same area or Level-1-2 routers in the same area. The Level-1 router maintains only the Level-1 LSDB. It calculates the network topology in the area and the optimal route to each network segment in the area according to the link state information contained in the LSDB. Level-1 routers must be connected to the IS-IS backbone network through Level-1-2 routers to access other areas.
Level-2: A Level-2 router can be considered as a backbone area router. It can establish IS-IS neighbor relationships only with Level-1-2 or Level-2 routers. A Level-2 router maintains only Level-2 LSDBs.
Level-1-2: The router can establish a neighbor relationship with a Level-1 router or establish a neighbor relationship with a Level-2 router. The role of the router is similar to that of an ABR in OSPF. It is usually connected to a Level-1 area and a backbone area. The Level-1 LSDB and Level-2 LSDB can be maintained at the same time. As a bridge between Level-1 areas and other areas, the Level-1 area delivers the Level-1 LSP and sets the ATT bit in the LSP to notify the Level-1 routers in the area to reach the external area of the area, the intra-area router generates a default route pointing to the Level-1-2 router according to the LSP with the ATT bit set.
Hierarchical IS-IS design: Backbone area and non-backbone area
A backbone area is a series of consecutive Level-2 and Level-1-2 routers. It is not unique.
A non-backbone area is a Level-1 area that consists of consecutive Level-1 routers (including Level-1-2 routers) and is also called a non-backbone area.
In the IS-IS network area design, the Level-1 area must be directly connected to the backbone area through a Level-1-2 router. The Level-1 area can be regarded as the Totally Stub area. By default, the Level-1-2 router does not learn the routes learned from the backbone area, routers in other areas advertise routes to the local Level-1 area.
