BGP Backdoor Attribute

Consider the above diagram, RTA and RTC are running EBGP, and RTB and RTC are running EBGP. RTA and RTB are running some kind of IGP (RIP, IGRP, etc.). By definition, EBGP updates have a distance of 20 which is lower than the IGP distances. Default distance is 120 for RIP, 100 for IGRP, 90 for EIGRP and 110 for OSPF.

RTA will receive updates about 160.10.0.0 via two routing protocols:

EBGP with a distance of 20 and IGP with a distance higher than 20.

By default, BGP has the following distances, but that could be changed by the distance command:

distance bgp external−distance internal−distance local−distance

external−distance:20
internal−distance:200
local−distance:200

RTA will pick EBGP via RTC because of the lower distance.

If we want RTA to learn about 160.10.0.0 via RTB (IGP), then we have two options:

• Change EBGP's external distance or IGP's distance, which is not recommended.
• Use BGP backdoor.
  

BGP backdoor makes the IGP route the preferred route.

Use the following network address backdoor command.

The configured network is the network that we would like to reach via IGP. For BGP this network will be treated as a locally assigned network except it will not be advertised in BGP updates.

RTA#
router eigrp 10
network 160.10.0.0
router bgp 100
neighbor 2.2.2.1 remote−as 300
network 160.10.0.0 backdoor

Network 160.10.0.0 is treated as a local entry, but is not advertised as a normal network entry.

RTA learns 160.10.0.0 from RTB via EIGRP with distance 90, and also learns it from RTC via EBGP with distance 20. Normally EBGP is preferred, but because of the backdoor command EIGRP is preferred.