IS-IS Terminology

IS-IS Terminology

A Router is an Intermediate System (IS) and a host is an End System (ES). The protocol that is used for communication between ES and IS is called ES-IS protocol, and the protocol that routers use to communicate with each other is called IS-IS protocol. ES-IS has no relevance to IS-IS for IP.

IS-IS Router Types:

There are three different types of IS-IS routers.

1) L1 routers: Level 1 routers are routers that have no direct connectivity with another area. These router maintain L1 link-state database. They are analogous to OSPF nonbackbone Internal routers.

2) L2 routers: Level 2 routers are routers that connect the areas. These routers maintain a L2 link-state database. These routers are analogous to OSPF backbone routers.

3) L1/L2 routers: L1/L2 routers are analogous to OSPF ABRs. These L1/L2 routers maintain a separate L1 link-state database and L2 link-state database. These routers can connect to L1 and L2 routers.

The set of L2 routers (including L1/L2 routers) and their interconnecting links is the IS-IS Backbone. Every L1 router within an area maintains a link-state database. L1/L2 routers do not advertise L2 routes to L1 routers.

To route a packet to another area, an L1 router must forward the packet to L1/L2 router .