What is BGP backdoor?

In BGP it is a component that is used to modify the admin distance of eBGP in order for an interior portal routing protocol (IGP) to take precedence over an eBGP route.

By default,External BGP (eBGP) has an admin distance value of 20. Administrative distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information for the same destination. Admin distance is a measure of the best path and reliability of the source of the routing information. The smaller the administrative distance value, the more reliable the protocol/link.

BGP selects a single path, by default, as the best path to a destination host or network. The best path selection algorithm analyzes path attributes to find out which route is installed as the best path in the BGP routing table. Each path carries well-known mandatory, discretionary, and optional transitive attributes that are used in BGP best path analysis.

The “Backdoor Feature” is often used to improve the administrative distance of eBGP to 200 with the goal of making the IGP learned routes to be preferred. A backdoor network is treated as a local network, except that it is not promoted. This is configured by using the network backdoor BGP command.

UK-R1(config)#router bgp 1010
UK-R1(config-router)#network 10.20.20.0 mask 255.255.255.0 backdoor