BGP – Modifying the AD of Backdoor networks

As you may know that the backdoor feature of BGP changes the AD to 200. But is there a way to modify it?

There is a way, it’s not very intuitive. When you configure a network as a backdoor network you are creating a local-route that you don’t originate to others. To modify a local-route you can use the distance bgp command. Remember that bgp distance format it:

distance bgp (eBGP) (iBGP) (Local/Backdoor)

Below is a quick example where I use the route 10.1.4.0/24 as a backdoor route.

router bgp 5
no synchronization
bgp log-neighbor-changes
network 10.1.4.0 mask 255.255.255.0 backdoor
neighbor 10.1.45.4 remote-as 4
distance bgp 20 200 233
no auto-summary

 

After modifying the distance make sure to clear that route to apply changes:

 

Router#clear ip route 10.1.4.0
Router#sh ip route | i 10.1.4.0
B       10.1.4.0 [233/0] via 10.1.45.4, 00:00:05

As you can see the distance of the backdoor network is 233 instead of the default 200.

When you change the AD of BGP local routes you are also changing the AD of any summary null routes when configuring aggregations.