Content

| 9.10 |  | BGP Redistribution |
| 9.10.1 |  | 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. Therefore, while a dynamic advertisement will end if the network being advertised no longer exists, a static advertisement will not. Each method has pros and cons, as will be seen 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. Dynamically injected information can be further divided into 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 of 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. This is 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. Controlling which of the IGP-learned routes are advertised by BGP is possible. Unfortunately, a network command is necessary for each route prefix, so when dealing with a large number of prefixes, maintaining semi-dynamic configurations is impractical. In fact, the Cisco IOS limits it to 200 network statements. Ultimately, a semi-dynamic configuration provides greater administrative control but dramatically increases administrative overhead.

BGP assumes that prefixes specified by the

network command exist in an IGP domain. This is verified 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, the no synchronization command can be used to disable this verification. However, in doing so, there is a risk that will allow a router to advertise networks that it cannot reach.