- 3.8.a [i] Single area, single topology
Small IS -IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.
Routers establish Level 1 adjacencies to perform routing within a local area (intra-area routing) . Routers establish Level 2 adjacencies to perform routing between Level 1 areas (inter-area routing). Some networks use legacy equipment that supports only Level 1 routing . These devices are typically organized into many small areas that cannot be aggregated due to performance limitations. Cisco routers are used to interconnect each area to the Level 2 backbone.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Location 4245). . Kindle Edition.