The router’s level capability can be configured globally and on a per-interface basis. The interface-level parameters specify the interface’s routing level. The neighbor capability and parameters define the adjacencies that are established.
IS-IS is not enabled by default. When IS-IS is enabled, the global default level capability is level 1/2 which enables the router to operate as either a level 1 and/or a level 2 router with the associated databases. The router runs separate shortest path first (SPF) calculations for the level 1 area routing and for the level 2 multi-area routing to create the IS-IS routing table.
The level value can be modified on both or either of the global and interface levels to be only level 1-capable, only level 2-capable or level 1 and level 2-capable.
If the default value is not modified on any routers in the area, then the routers try to form both level 1 and level 2 adjacencies on all IS-IS interfaces. If the default values are modified to level 1 or level 2, then the number of adjacencies formed are limited to that level only.