Redistributing Routes into BGP

This topic describes route redistribution in BGP and identifies the Cisco IOS commands that are required to configure BGP route redistribution.

There are two alternatives for injecting local routes into the BGP table: list them using the network command or redistribute them. Listing the routes gives you total control over networks that could possibly be advertised by BGP. This option is very desirable for multihomed customers or Internet service providers (ISPs). On the other hand, this approach requires a lot of configuration commands that could be hard to maintain.

If there are a lot of networks to be advertised, and BGP is used primarily to achieve scalability, not routing security (for example, in enterprise networks), it could be easier to let the local IGP find the routes and then redistribute them into BGP. However, this approach introduces the risk that the IGP may find some networks that are not supposed to be advertised. Private network numbers, such as network, are often used within an AS for various reasons but must never be advertised out to the Internet. Careful filtering must be done to prevent unintentional advertising.

When the router injects a route that is listed with a network command into its BGP table, the origin code is set to "IGP." If the route is injected into the BGP table through redistribution, the origin code is set to "unknown/incomplete."

© 2005, Cisco Systems, Inc. BGP Overview 1-73

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