network x.x.x.x mask y.y.y.y backdoor

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#bgpbackdoor

Ref Task 3.5 in IEWB vol 2 lab6

backdoor is used when you have some IGP advertising the same network also learnt via eBGP
Since the AD of eBGP is 20 lowest compared to all other IGPs

We are left with two choices
1) lower the AD of IGP to 19
2) use the backdoor.

Referring to the link above, I found out the backdoor cmd in bgp

What this does is make the network added to that router seem like it's LOCAL
At the same time, router doesnt advertise this route to any IBGP/eBGP peers.

So it treats this route as [200/x] (x is the metric)
==>IBGP learnt route.
Now any IGP advertising this route can instill this network into the routing table by the virtue of it having the lowest AD.