BGP Backdoor

R1 configuration

interface FastEthernet0/0  ip address 10.1.12.1 255.255.255.0  duplex auto  speed auto ! interface FastEthernet0/1  ip address 10.1.13.1 255.255.255.0  duplex auto  speed auto ! router bgp 100  no synchronization  bgp log-neighbor-changes  neighbor 10.1.12.2 remote-as 200  neighbor 10.1.13.3 remote-as 300  no auto-summary

R2 configurations

interface Loopback0  ip address 2.2.2.2 255.255.255.0 ! interface FastEthernet0/0  ip address 10.1.12.2 255.255.255.0  duplex auto  speed auto ! interface FastEthernet0/1  ip address 10.1.23.2 255.255.255.0  duplex auto  speed auto !         router eigrp 100  network 2.2.2.2 0.0.0.0  network 10.1.23.2 0.0.0.0  auto-summary ! router bgp 200  no synchronization  bgp log-neighbor-changes  network 2.2.2.0 mask 255.255.255.0  neighbor 10.1.12.1 remote-as 100  no auto-summary

R3 configuration

interface FastEthernet0/0  ip address 10.1.13.3 255.255.255.0  duplex auto  speed auto ! interface FastEthernet0/1  ip address 10.1.23.3 255.255.255.0  duplex auto  speed auto ! router eigrp 100  network 10.1.23.3 0.0.0.0 no auto-summary ! router bgp 300  no synchronization  bgp log-neighbor-changes  neighbor 10.1.13.1 remote-as 100  no auto-summary

Now let’s check the routing table on R3 for 2.2.2.0 the prefix that we advertised on both EIGRP and BGP on R2

R3#sh ip route 2.2.2.0 Routing entry for 2.2.2.0/24   Known via "bgp 300", distance 20, metric 0   Tag 100, type external   Last update from 10.1.13.1 00:02:15 ago   Routing Descriptor Blocks:   * 10.1.13.1, from 10.1.13.1, 00:02:15 ago       Route metric is 0, traffic share count is 1       AS Hops 2       Route tag 100

so as we can see R3 is using R1 to reach 2.2.2.0/24 on R2 this is because the AD of EBGP is 20 and the AD of EIGRP is 90 the EBGP route was preferred over the EIGRP one

one way to fix this is to change the AD of that route on R3 to something higher than

to do this we need to match the prefix with an access-list  and then we assign a higher distance under the BGP configuration on R3

R3(config)#access-list 1 permit 2.2.2.0 0.0.0.255 R3(config)#router bgp 300 R3(config-router)#distance 95 10.1.13.1 0.0.0.0 1

Now let’s  check the 2.2.2.0/24 route on R3

R3(config)#do sh ip route  2.2.2.0 Routing entry for 2.2.2.0/24   Known via "eigrp 100", distance 90, metric 409600, type internal   Redistributing via eigrp 100   Last update from 10.1.23.2 on FastEthernet0/1, 00:05:03 ago   Routing Descriptor Blocks:   * 10.1.23.2, from 10.1.23.2, 00:05:03 ago, via FastEthernet0/1       Route metric is 409600, traffic share count is 1       Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit       Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 1

As we can see R3 is now using EIGRP to reach that prefix on R2

Another way of doing this is to use the backdoor option in BGP as changing AD is not recommended in BGP real world as it may cause routing loops

R3(config-router)# network 2.2.2.0 mask 255.255.255.0 backdoor

Now let’s the 2.2.2.0/24 on R3

R3(config-router)#do sh ip route 2.2.2.0 Routing entry for 2.2.2.0/24   Known via "eigrp 100", distance 90, metric 409600, type internal   Redistributing via eigrp 100   Last update from 10.1.23.2 on FastEthernet0/1, 00:02:35 ago   Routing Descriptor Blocks:   * 10.1.23.2, from 10.1.23.2, 00:02:35 ago, via FastEthernet0/1       Route metric is 409600, traffic share count is 1       Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit       Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 1