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BGP Backdoor Network feature

We may come across on interesting issue if we get the same prefix from external BGP AS and directly via OSPF process. It may happen if we want to have another link to the destination but not neccesarily via another eBGP link. The problem that we will definetely come across on is just  another variation of suboptimal routing. 

Let’s have a look on below network diagram and assume that we are an administrator in AS2. We have BGP neighboorship with AS1 and we share OSPF Area0 with R3. The OSPF link  may be just “super fast” Internet link with using of GRE tunnels or direct connectivity alike in our case. Router R3 advertises prefix via BGP and OSPF. R2 gets this prefix from R1 and directly from R3. So far nothing fancy, but if we take into consideration that there is no router in the  middle on the OSPF path and there is a BGP router on BGP path we don’t have to do calculation to come to the proper conclusion that prefered path should be OSPF. Unfortunately AD of eBGP route is 20 and OSPF 110 so we will get this prefix via BGP. We may have a couple of AS on the path before it reach our AS.  This is just another example of “suboptimal routing” that we would like to avoid.

BGP Backdoor network

Can we do something about that to provide desirable path to the network via OSPF? Yes, we have to use cool BGP feature called “BGP Backdoor“. First, let’s check what  we have in the routing table before utilization of “BGP backdoor” feature on R2 router.



As we see R2 router knows about thanks to OSPF (show ip ospf route) but it has chosen BGP according to the lower AD (show ip route). Now, let’s use BGP backdoor with one simple command

router bgp 2
network mask backdoor

Now, if we check the RIB table we will see that to get prefix R2 uses OSPF link with next hop of and the paths looks in this way:

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