Migrating Between Routing Protocols
This section discusses two common approaches for migrating between routing protocols. One approach for migrating between routing protocols is to use administrative distance (AD) to migrate the routing protocols. Another approach is to use redistribution and a moving boundary.
Migration by AD does not use redistribution. Instead, two routing protocols are run at the same time with the same routes. This assumes sufficient memory, CPU, and bandwidth are in place to support this on the routers running two routing protocols.
The first step in migration by AD is to turn on the new protocol, but make sure that it has a higher AD than the existing routing protocol so it is not preferred. This step enables the protocol and allows adjacencies or neighbors and routing databases to be checked but does not actually rely on the new routing protocol for routing decisions.
When the new protocol is fully deployed, various checks can be done with show commands to confirm proper deployment. Then, the cutover takes place. In cutover, the AD is shifted for one of the two protocols so that the new routing protocol will now have a lower AD.
Final steps in this process include the following:
- Check for any prefixes learned only via the old protocol.
- Check for any strange next hops (perhaps using some form of automated comparison).
With migration by redistribution, the migration is staged as a series of smaller steps. In each step, part of the network is converted from the old to the new routing protocol. In a big network, the AD approach might be used to support this conversion. In a smaller network, an overnight cutover or simpler approach might suffice.
To provide full connectivity during migration by redistribution, the boundary routers between the two parts of the network would have to bidirectionally redistribute between protocols. Filtering via tags would be one relatively simple way to manage this. The boundary routers move as more of the region is migrated.