IS-IS is a dynamic routing protocol that operates within an autonomous system (or domain). IS-IS provides support for hierarchical routing, which enables you to partition large routing domains into smaller areas. When used separately from SPBM, IS-IS uses a two-level hierarchy, dividing the domain into multiple Level 1 areas and one Level 2 area. When used separately from SPBM, the Level 2 area serves as backbone of the domain, connecting to all the Level 1 areas. SPBM currently uses only Level 1 areas.
Important
The IEEE 802.1aq standard currently only defines the use of one hierarchy, Level 1. Level 2 function is disabled.