BGP backdoor it’s a well-known feature of the BGP which is used to change the AD (administrative distance) of eBGP. By default, external BGP (eBGP) has an administrative distance value of 20 with the help of a backdoor command you can set 200 AD. If two routing protocols is providing route information for the same destination the administrative distance is the first criterion that a router uses to determine which routing protocol to use for the best path. The lowest AD value is a more reliable protocol and link.
Why do we need to change eBGP AD?
Whenever our router learns about a network (prefix) through eBGP and also with an IGP protocol like OSPF EIGRP or RIP then our router always chooses Ebgp route because Ebgp uses an administrative distance value of 20 so our router by default preferred eBGP over EIGRP AD 90, RIP AD 120, OSPF AD 110.
In some scenarios this becomes a problem let’s see the configuration.
- configure the topology as per the diagram.
- assign the IP addresses
- configure EIGRP 100 on router 1 and router 3
- advertise the interfaces
- configure eBGP peering between router 1 and 2
- configure eBGP peering between router 2 and router 3
- make sure router 1 gets the 192.168.30.1 route from serial 4/3 link via EIGRP.
- configure backdoor in order to get 192.168.30.1 route via serial link 4/3 from router 3.