9.5 Configure BGP to Interact with IGPs
9.5.1
Injecting information dynamically into BGP
In networks as complex as today's Internet, route stability is a big issue. There is a close correspondence between route fluctuations and the stability of the Internet access links on one hand and how the routing information was injected into the Internet via BGP on the other hand. 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 they identify. Statically injected routes are constantly maintained by the BGP routing tables, regardless of the status of the networks they identify. Thus, while a dynamic advertisement will cease if the network being advertised no longer exists, a static advertisement would not. Each method has its pros and cons, as you will see next.

Dynamically injected information can be further divided into purely dynamic, where all the IGP routes are redistributed into BGP (redistribute command), and semidynamic, where only certain IGP routes are to be injected into 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 to be redistributed into BGP. A variety of IGPs are currently used in autonomous systems, including Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced IGRP (EIGRP), Open Shortest Path First (OSPF), and Intermediate System-to-Intermediate System (IS-IS). What dynamic redistribution offers is ease of configuration: all internal IGP routes will dynamically flow into BGP, regardless of the protocols being used.

A semidynamic 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 less than completely dynamic because a list of all the prefixes that need to be advertised must be maintained in the router-the router is not automatically injecting all IGP routes into BGP. If the list of prefixes is large, maintaining it would be impractical. Besides, router implementations might put an upper limit on the number of prefixes that can be listed. If the number of prefixes to be advertised is larger than that upper-bound limit, dynamically redistributing the IGP into BGP is a must.

BGP assumes that networks defined with the network command are existing networks and will try to verify that by checking in the IP routing table. If BGP does not find an exact match for these networks, they will not be advertised. The verification step is smart because advertising a network not connected to or unknown by the router is misleading to other networks because the router cannot deliver traffic to this network anyway.

Injecting routes into BGP via the network command offers a more controlled way of route advertisement. Injecting IGPs into BGP via redistribution could result in the side effect of leaking unwanted or faulty information into BGP, as you will see next.