Intermediate System - Intermediate System -- Part 1

ISIS was originally designed for Open System Interconnect (OSI) protocol Suite. The Connectionless Network Service (CLNS) is been used & introduced by OSI.

The form of running on Cisco routers is Integrated IS-IS, Which supports both CLNP and IP.

ISIS Terminology

An OSI domain is very much like a domain with the other routing protocols you've studied – it's a segment of a network that's under a common administrative control. An area is a logical segment of the network that is composed of contiguous router and their connecting data links.

These are logical entities, but there are two physical bodies as well. A host is an End system (ES). An Intermediate System (IS) is a router. Not a group of routers, a single router.

ISIS uses a two level hierarchy, allowing a large domain to be subdivided into areas

An L1/L2 router can act as an L1 and L2 router at the same time. An L1/L2 router can have neighbor in separates ISIS areas. The L1/L2 router will have two separate database – one for L1 routes and another for L2 routes

L1/L2 is the default setting for Cisco routers running ISIS

In Upcomming parts will digg further.

ISIS has three type of router: - Level 1 [L1], Level 2[L2] and L1/L2 

Level -1

L1 routers are contained in a single area, and are connected to other areas by an L1/L2 router. The L1 uses the L1/L2 router as a default gateway to reach destination contained in other areas.

Level - 1/2

An L1/L2 router can act as an L1 and L2 router at the same time. An L1/L2 router can have neighbor in separates ISIS areas. The L1/L2 router will have two separate database – one for L1 routes and another for L2 routes

L1/L2 is the default setting for Cisco routers running ISIS