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1.2.a [iii] Routing protocol migration
There are two common approaches for migrating between routing protocols.
Use administrative distance (AD) to migrate the routing protocols
Use redistribution and a moving boundary
When using migration by AD method, two routing protocols are run at the same time. This approach assumes sufficient resources such as memory, CPU, and bandwidth are in place. 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 older routing protocol so it is not preferred. This step enables the protocol and allows adjacencies or neighbors and routing databases to be formed 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 that everything is working as desired. During the final cutover, the AD is shifted for one of the two protocols so that the new routing protocol will now have a lower AD hence preferred. With migration by redistribution method, the migration is completed as a series of smaller steps. In each step , part of the network is converted to the new routing protocol. In a big network, the AD approach might be used to support this conversion. In a smaller network, an immediate cutover might suffice . To provide full connectivity during migration by redistribution, the boundary routers between the two parts of the network would have to bi-directionally redistribute between the two routing protocols. Filtering via tags would be one effective way to manage this situation.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1244-1253). . Kindle Edition.