ISIS part 3

ISIS knows 2 types of addresses which are NSAP (Network Service Access Point) address and NET (Network Entity Title) address. The NSAP address is the layer 2 (network layer) address for CLNS packets. The majority of routers have only 1 NSAP address. The NET address is the address of the host router and has the same fields as NSAP address. The difference is that NET address has the network selector bit set to 0.

The NSAP/NET address has 3 major segments which are: areaID.systemID.NSEL.

The area ID is used to route between the areas (L2 routing), and contains 2 or more digits. The area ID must be the same for every router in the same area. This address contains of AFI (Authentication and Format ID) to identify the assigning authority, IDI (Inter-Domain ID) to identify the domain, and High Order DSP to identify sub domain/area. Routers will check the area ID upon arriving packets and determines whether the area ID is the same. If it is the same, then it does an L1 routing. If the area ID is different, it will send the packet to an L1/L2 router where it will send the packet to another area.

The system ID is used to route routers or hosts in an area (L1 routing), and contains exactly 12 digits. Every L2 router must have a unique system ID in the entire domain, while every L1 router must have a unique system ID in the entire area.

The NSEL (Network Selector) field is used to do the routing within the ES (on a router it is marked by 0x00), and contains 2 digits. If the NSEL is set to 00, then it is the NET address instead of NSAP address.

A basic router setup using ISIS is as below:

(config)#int

(config-if)#ip router isis

(config)# router isis

(config-router)#net AA.BBBB.BBBB.BBBB.BBBB.CC (A=Area ID, B=System ID, C=NSEL).

IP router isis has to be set on every interface in the ISIS process. If a new router comes to the area, the new router interface to the existing neighbor router has to configure the router isis and net A.B.C command. In the meanwhile, on the neighbor router the router isis command needs to be applied to the interface connected to the new router.

-- 1 March 2011 --