BGP Backdoor

Configure BGP R1, R2 & R3
R1
router bgp 100
net 1.1.1.0 mask 255.255.255.0
neigh 10.1.12.2 remote-as 200
neigh 10.1.13.3 remote-as 300

R2
router bgp 200
net 2.2.2.0 mask 255.255.255.0
neigh 10.1.12.1 remote-as 100

R3
router bgp 300
net 3.3.3.0 mask 255.255.255.0
neigh 10.1.13.1 remote-as 100

Configure EIGRP Between R2 and R3
R2
router ei 100
net 22.22.22.0 mask 255.255.255.0
net 10.1.23.0 mask 255.255.255.0
no au

R3
router ei 100
net 33.33.33.0 mask 255.255.255.0
net 10.1.23.0 mask 255.255.255.0

Add lo1 to BGP on R2 & R3
R2
router bgp 200
net 22.22.22.0 mask 255.255.255.0

R3
router bgp 300
net 33.33.33.0 mask 255.255.255.0

After above configuration R2 use 10.1.12.0 link for 33.33.33.33

Before Backdoor R2 use 10.1.12.0 for 33.33.33.33 cause this link’s AD is 20, 10.1.23.0 link is EIGRP 90
R2#
     1.0.0.0/24 is subnetted, 1 subnets
B       1.1.1.0 [20/0] via 10.1.12.1, 00:02:07
     2.0.0.0/24 is subnetted, 1 subnets
C       2.2.2.0 is directly connected, Loopback0
     33.0.0.0/24 is subnetted, 1 subnets
B       33.33.33.0 [20/0] via 10.1.12.1, 00:00:16
     3.0.0.0/24 is subnetted, 1 subnets
B       3.3.3.0 [20/0] via 10.1.12.1, 00:02:07
     22.0.0.0/24 is subnetted, 1 subnets
C       22.22.22.0 is directly connected, Loopback1
     10.0.0.0/24 is subnetted, 2 subnets
C       10.1.12.0 is directly connected, Serial0/0
C       10.1.23.0 is directly connected, FastEthernet1/0


After Backdoor R2 use 10.1.23.3 for 33.33.33.33 even though AD 90 is higher than 20…
R2#
     1.0.0.0/24 is subnetted, 1 subnets
B       1.1.1.0 [20/0] via 10.1.12.1, 00:03:13
     2.0.0.0/24 is subnetted, 1 subnets
C       2.2.2.0 is directly connected, Loopback0
     33.0.0.0/24 is subnetted, 1 subnets
D       33.33.33.0 [90/156160] via 10.1.23.3, 00:00:25, FastEthernet1/0
     3.0.0.0/24 is subnetted, 1 subnets
B       3.3.3.0 [20/0] via 10.1.12.1, 00:03:13
     22.0.0.0/24 is subnetted, 1 subnets
C       22.22.22.0 is directly connected, Loopback1
     10.0.0.0/24 is subnetted, 2 subnets
C       10.1.12.0 is directly connected, Serial0/0
C       10.1.23.0 is directly connected, FastEthernet1/0

R2#tra 33.33.33.33

Type escape sequence to abort.
Tracing the route to 33.33.33.33

  1 10.1.23.3 4 msec 28 msec 24 msec
R2#