ISIS part 1

IS-IS (Intermediate System to Intermediate System) is one of the routing protocols using the SPF/Dijkstra algorithm. ISIS has an AD of 115 and has a default metric of 10. ISIS itself is originally designed to work at layer 2 (network layer) using CLNS (Connectionless Network Service), while its layer 3 routing protocol uses CLNP (Connectionless Network Protocol). There are some key terminologies in ISIS protocol. The protocol is divided into logical entities (domain and area) and physical bodies (end system –ES = host, and intermediate system-IS = router).

There are 3 types of ISIS routers:

1. L1 (Level 1) router which is placed in a single area. This router has only a database of the neighboring L1 routers. An L1 router can only connect to another L1 router in the same area. If this router wants to connect to an L1 router in another area, is has to through a L1/L2 router as its default gateway.

2. L2 (Level 2) router which is placed into border areas. This router can be used to route between different areas. L2 routers has only a database of the neighboring L2 routers and therefore can connect to only L2 routers in different border areas. If it wants to go to a L1 router, it has to connect to a L1/L2 router.

3. L1/L2 router is a router that can act as a L1 router (intra area) and L2 router (inter area). Therefore, this router has 2 databases, each for L1 routes and L2 routes. The L1/L2 router together with the L2 router makes sending data possible to other areas. The L1/L2 router can connect to another L1/L2 area in any area, to L2 router in any area, and to L1 router in the same area.

The L1 router acts as an access router, where the backbone routers consist of L1 /L2 routers, L2 routers or a combination of both. L1 has different hello packets than that of L2, and maintains adjacency with its neighboring L1 router only. The only possible way for an L1 router to communicate with an L2 router is through the L1/L2 routers (gateway). Changing the levels is possible, and can be done on the interface (locally) or on the router (globally).

To configure this on the interface use the command:

(config)#interface

(config-if)#isis circuit-type

To configure this on the router use the command:

(config)#router isis

(config-router)#is-type

To see the levels on the neighboring routers use the (config)#show clns neighbor command.

ISIS can do route summarization. The routes can be shown using the #show ip route isis command. This route summarization needs to be configured on the border router (L1/L2 or L2 router), else the summarization will not work. If there are multiple routes from an isis area to another isis area (load balancing), both of the border routers needs to have the same summary routes, else all the traffic goes to the non-summarized router.

-- 1 March 2011 --