Networks develop and grow over time. Subnets are split up, more equipment and new routes are added, and router configurations become more complex. Changing the IGP of an active network is nothing short of a monumental task—and has the potential to be a nightmare when it comes to preserving high availability.
Five general steps are involved in migrating to another IGP:
Plan for the migration. Arguably the most important step, proper planning can make or break the entire migration.
Add the new protocol to the network. The new protocol is added (as the less preferred protocol), mirroring the existing one.
Make the new protocol “preferred.” Once the new protocol is in place it can be made the preferred protocol, effectively taking over from the original protocol.
Verify the success of the migration. When the new protocol is active it is essential to check the results and look for any problems or issues.
Remove the original protocol. After the new protocol is in a stable state in the network, the original protocol can be removed.
Before any migration takes place, it is important to have a good sense of what your current network looks like. In fact, documenting the details of the entire network might be the most important step in the migration!
This section uses the following example to help illustrate the steps of a successful migration:
show routeinet.0: 9 destinations, 9 routes (9 active, 0 holddown, 0 hidden) + = ...