distance, the IBGP route is propagated to router Philly because synchronization is turned
on.
Configuring Backdoor Routes
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 on page 137.
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 21 on page 134,
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 on page 137, 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
• Use to cause a backdoor IGP route to be preferred over an EBGP route to the same
prefix by setting the administrative distance of the EBGP route to that of an IBGP route,
200.
• Issuing this command does not cause the BGP speaker to advertise the specified route.
• This command takes effect immediately.
137Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring BGP Routing
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