In certain network topologies, a BGP speaker might learn routes to the same prefix from an external BGP peer and by means of an IGP protocol. Consider the network structure shown in Figure 40.
A company has established an OSPF link between routers NY and Boston. This private link between the two routers is known as a backdoor link. Router NY learns two routes to prefix 172.19.0.0/16; one by means of OSPF from router Boston, and one by means of EBGP from router LA through router SanDiego. As was shown in Table 22, EBGP routes have an administrative distance of 20 and are preferred over IGP routes, which have much higher administrative distances. In this example, the longer path by means of EBGP is preferred over the OSPF backdoor path with its distance of 110.
Figure 40: Backdoor Route
You can modify this behavior by issuing the network backdoor command on router NY:
- host1(config)#router bgp 300
- host1(config-router)#neighbor 10.4.4.1 remote-as
400
- host1(config-router)#network 172.19.0.0 backdoor
Unlike the typical network command, network backdoor does not cause the BGP speaker to advertise the specified prefix. Instead, it sets the administrative distance for the EBGP path to that prefix to the same value as a route learned by means of IBGP. That is, the EBGP administrative distance is changed from the highly preferred value of 20 to the highly unpreferred value of 200. In Figure 40, this change in value results in the backdoor OSPF being more preferred as a way to reach prefix 172.19.0.0/16.
network backdoor
- host1(config-router)#network 10.53.42.0 backdoor
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