Some situations call for migrating between routing protocols, for example, where one company is purchased by a larger company that has a security policy that prevents the use of any proprietary protocols anywhere in the organization. If the purchased company previously used EIGRP, it would have to migrate quickly to OSPF to adhere to the new organization’s policy. There are two ways to migrate between routing protocols:
- Using AD: This involves turning on the new protocol (OSPF in the example above) and then making certain that it has a higher AD than the existing EIGRP routing protocol. This procedure enables OSPF and allows adjacency and the routing databases to be referred to, but it will not actually rely on the OSPF routing protocol for routing decisions. When OSPF is fully deployed, the AD can be changed for one of the two protocols so that OSPF will not have the lower AD.
- Performing redistribution by moving the boundary routers in small steps: With migration by redistribution, in each step of the process a different part of the network will be converted from one routing protocol to another (EIGRP to OSPF in the example above).
To migrate between routing protocols in a large enterprise organization, the first approach (changing the AD) is often the best option. To achieve full connectivity using the second option (migration by redistribution), the boundary routers between the two parts of the networks will have to be bidirectionally redistributed between the two routing protocols. Then, filtering can be implemented using the tagging procedure mentioned previously. After this process, the boundary routers will “move” as more of the organization is migrated to the new routing protocol.