Hello Ankit
Say you have two enterprise networks connected to an ISP, and they are exchanging routing information via eBGP. Now imagine that these two enterprises are sister companies, and they have arranged to have an independent link between their campuses for internal network connectivity for sharing applications and other network resources. Let’s that these two companies are sharing routing information via this link using OSPF or EIGRP. Under normal circumstances, any routing between these two companies would take place via the eBGP links over the ISP because eBGP has an AD of 20 while OSPF and EIGRP have ADs of 110 and 90 respectively.
Now in order to resolve this routing issue, the “backdoor” feature is used. What this does is it increases the administrative distance of eBGP to 200 with the goal of making the IGP learned routes to be preferred. This allows all traffic between the enterprise sites to be routed via the private link rather than via the ISP. Note that a backdoor network is treated as a local network, except that it is not advertised.
I hope this has been helpful!
Laz