IS-IS General Info

Intermediate system to Intermediate System (IS-IS). Interior Gateway Protocol (IGP).

Router = Intermediate System (IS)
PC= End-System (ES).

IS-IS is router to router communication.

Connectionless Network Protocol (CLNP) used for Connectionless Network Service (CLNS), network layer of OSI model.

IS-IS uses a CLNS address as router ID, and to group routers into areas, address is administrative only.

OSI supports four levels of routing:

Level 0 - End systems, end system to Intermediate system
Level 1 - Exchange routes within an area
Level 2 - Backbone between areas
Level 3 - AS to AS (inter-domain routing protocol IDRP)

IS-IS operates on levels 1/2 within and between areas.