5. Cisco ISIS Configuration Example with Single Area

To start, consider the topology and basic configuration of the ISIS protocol. There are some points that are important to notice in this scenario:

• All routers in this scenario are in the same area. In the next section we will implement another scenario where there is more than one area.
• To enable the ISIS routing protocol in the router, we use the command “router isis“.
• To activate ISIS routing protocol in interfaces, unlike all other IGP protocols, we do not use the network command, but we use the “ip router isis” command in the context of the interface itself.
• As already mentioned, all ISIS routers must have a CLNS address, or more accurate a NET address. The format of the NET address has already been discussed in the previous section.
• In this scenario, there is only one area and the area number is 0001. The system ID is a unique 6-byte value for each router. You can check the system ID of each router with the “show isis hostname” command. The NSEL value is always 00.
• By default, all ISIS routers and interfaces are of type L1/L2. In the scenario where there is only one area, either L1 or L2 type is enough to be configured and there is not much difference between L1 and L2. Therefore, in this scenario, with the command “is-type level-1“, all routers are configured as L1.
• When the router is-type as only L1 or L2 is configured, then the level of all of its interfaces are also determined according to the router is-type.
• In this scenario, if you leave all routers as L1/L2, no problem will arise, but two separate neighbors will be created, one with L1 and the other with L2. Also two separate databases L1 and L2 are created in each of the routers. Therefore the resources are not used optimally.