Case Study: Migration of Areas

Although Integrated IS-IS requires each router to be in only one area, multiple NSAPs addresses (each with a different area ID) can be used simultaneously for the purpose of renumbering , splitting, or merging of areas. Using multiple NSAPs on an IS-IS router is also known as multihoming. In the default mode of operation, Cisco IOS allows up to three NSAP addresses, each with a different area ID but the same system ID. It should be stressed that configuring a router with more than one area address must be for only transitional situations. Multihoming is covered in detail in Chapter 4, "Addressing in Integrated IS-IS," in the "Configuring Multiple Nets for a Single IS-IS Process" section.

As you learned in the previous chapters, for IS-IS routers to form a Level 1 adjacency , they must have at least one area common address.

Figure 8-1 shows the migration of the four routers in area 49.0001 to area 49.0002.

Figure 8-1. Area migration.

The following elaborates how each of the routers can be migrated . We start with a simplified sample configuration as follows :

 router isis  net 49.0001.0000.0000.0001.00 

The first step is to apply the second NSAP address with the new area prefix 49.0002 to each of the routers as shown here. Each router has its own system ID, which does not change in the new address:

 router isis  net 49.0001.0000.0000.0001.00    net 49.0002.0000.0000.0001.00 

During the migration, multiple adjacencies are not created. However, each router is aware that it has a number of NSAP addresses, placing it in multiple areas.

The final step is to remove the NSAP with the old area ID, 49.0001, from all the routers, leaving the NSAP with the new area ID as shown here:

 router isis  net 49.0002.0000.0000.0001.00 

This completes the migration process. During the migration, no loss of adjacency occurs, and, therefore, there is no loss of connectivity or service disruption.