Vaibhav Singh

This topic is about BGP’s ability to prefer another router (kinda de-prefer, actually). The following diagram will make it clear.

In this situation, it may be a better choice to traverse from R1 to R2 via the point-to-point link instead of going over the ISP Cloud. Here R1 and R2 are eBGP peers only. The P2P link is peered over OSPF. But, since EBGP AD is 20 and OSPF 110. The route via ISP will be preferred for Lo5 network to reach Lo15 network segment.

Configuration

!--R1

R1#sh ip int brief | ex un
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet1/0            172.16.126.1    YES manual up                    up   
Serial2/0                  10.201.1.1      YES manual up                    up   
Loopback5                  172.16.5.1      YES manual up                    up     

router ospf 100
 log-adjacency-changes
 redistribute bgp 100
 network 172.16.5.0 0.0.0.255 area 0
 network 172.16.126.0 0.0.0.255 area 0

router bgp 100
 no synchronization
 bgp log-neighbor-changes
 network 10.201.1.0 mask 255.255.255.252
 network 172.16.6.0 mask 255.255.255.0
 network 172.16.16.0 mask 255.255.255.0 backdoor
 neighbor 10.201.1.2 remote-as 200
 no auto-summary

!--R2

R2#sh ip int brie | ex un
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet1/0            172.16.126.2    YES manual up                    up   
Serial2/0                  10.201.1.5      YES manual up                    up   
Loopback15                 172.16.15.1     YES manual up                    up     

router ospf 100
 log-adjacency-changes
 redistribute bgp 300
 network 172.16.15.0 0.0.0.255 area 0
 network 172.16.126.0 0.0.0.255 area 0

router bgp 300
 no synchronization
 bgp log-neighbor-changes
 network 10.201.1.4 mask 255.255.255.252
 network 172.16.6.0 mask 255.255.255.0 backdoor
 network 172.16.16.0 mask 255.255.255.0
 neighbor 10.201.1.6 remote-as 200
 no auto-summary

R2#sh ip route
Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 5 subnets
S       172.16.16.0 is directly connected, Null0
C       172.16.15.0 is directly connected, Loopback15
B       172.16.5.0 [20/0] via 10.201.1.6, 00:00:23
B       172.16.6.0 [200/0] via 10.201.1.6, 00:13:48
C       172.16.126.0 is directly connected, FastEthernet1/0
     10.0.0.0/30 is subnetted, 2 subnets
B       10.201.1.0 [20/0] via 10.201.1.6, 00:25:59
C       10.201.1.4 is directly connected, Serial2/0

Conclusion

The above output shows the difference in AD of the 5.x and 6.x networks coming from R1. When 6.x network was announced as a BGP backdoor its AD was raised to 200.