9.6 BGP Redistribution
9.6.1 BGP redistribution overview
When dealing with BGP, route stability is an important issue. There is a close relationship between Internet route stability and the method used to inject routes into BGP. Information can be injected into BGP dynamically or statically. Dynamically injected routes come and go from the BGP routing table, depending on the status of the networks that they identify. Statically injected routes are constantly maintained by the BGP routing tables, regardless of the status of the networks that they identify. Thus, while a dynamic advertisement will cease if the network being advertised no longer exists, a static advertisement will not. Each method has pros and cons, as you will see in this section.

Dynamically injected information can be further divided into purely dynamic redistribution, in which all the IGP routes are redistributed into BGP using the redistribute command, and semi-dynamic redistribution, in which only certain IGP routes are to be injected into BGP using the BGP network command. The distinction reflects both the level of user intervention and the level of control in defining the routes to be advertised.

Information is injected dynamically into BGP by enabling all the IGP routes (RIP, OSPF, EIGRP, and so on) to be redistributed into BGP. Dynamic redistribution offers ease of configuration: All internal IGP routes dynamically flow into BGP, regardless of the protocols being used.

The semi-dynamic method of injecting information into BGP is to specify a subset of IGP networks to be advertised by individually listing them for injection into BGP by using the network command. This method is more selective than a completely dynamic method. You control which of the IGP-learned routes are advertised by BGP. Unfortunately, a network command is necessary for each route prefix, so if you are dealing with a large number of prefixes, maintaining a semi-dynamic configuration is impractical. In fact, the Cisco IOS limits you to 200 network statements. Ultimately, a semi-dynamic configuration provides greater administrative control but dramatically increases administrative overhead.

As you have seen, BGP assumes that prefixes specified by the network command exist in an IGP domain and will verify this by checking for them in the routing table. If an IGP has not learned about a local route, BGP will not advertise it. Of course, you can use the no synchronization command to disable this verification, but in doing so, you risk allowing a router to advertise networks that it can not reach.