BGP Backdoor Routes

The backdoor option of the network BGP router subcommand changes the administrative distance of EBGP to allow IGP routes favored over EBGP routes for specific network numbers.

The figure above shows that AS 1 and AS 2 are running an IGP (EIGRP) on the private link between them, and are running EBGP with AS 3. RT1 in AS 1 will receive advertisements about 192.168.1.0/24 from AS 3 via EBGP with an administrative distance of 20 and from AS 2 via EIGRP with an administrative distance of 90. RT1 will use the EBGP route via AS 3 to reach 192.168.1.0/24 as the lower administrative distance is preferred.

Below shows that implementing the network 192.168.1.0 backdoor BGP router subcommand on RT1 changes the administrative distance of the EBGP route 192.168.1.0/24 from 20 to 200, and eventually makes the EIGRP route with an administrative distance of 90 more preferred. Note that the command will not cause BGP on RT1 to generate an advertisement for that network.

RT1#sh ip route

Gateway of last resort is not set

     12.0.0.0/24 is subnetted, 1 subnets
C       12.12.12.0 is directly connected, FastEthernet0/0
B    192.168.1.0/24 [20/0] via 13.13.13.3, 00:00:10
     13.0.0.0/24 is subnetted, 1 subnets
C       13.13.13.0 is directly connected, FastEthernet1/0
RT1#
RT1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
RT1(config)#router bgp 1
RT1(config-router)#network 192.168.1.0 backdoor
RT1(config-router)#end
RT1#
RT1#sh ip route

Gateway of last resort is not set

     12.0.0.0/24 is subnetted, 1 subnets
C       12.12.12.0 is directly connected, FastEthernet0/0
D    192.168.1.0/24 [90/156160] via 12.12.12.2, 00:00:05, FastEthernet0/0
     13.0.0.0/24 is subnetted, 1 subnets
C       13.13.13.0 is directly connected, FastEthernet1/0
RT1#