BGP Backdoor Routes

This topic is to discuss the following lesson:

excellent and very helpful note . easily to understand . for all u articles… i’n new to your website but really i thanksfull for your great effort. .

Thanx a lot Rene … Excellent explanation … It was so systematic … to the point … and U made it so easy to understand … Great work

Great topic. Thanks Rene

Excellent! Thank you Rene! Can we assume that R1 and R2 are remote offices interconnected via L2 MPLS and the BGP peering is used for redundancy?

Hi Marty,

That could be possible yes. MPLS L3 VPN is also possible, perhaps a topology like this:

MPLS L3 VPN Configuration


Hi Rene,
when we write

router bgp 100
network mask backdoor

is that mean changing the AD to 200 for all network or only default route?

Why did the AD go from 20 to 200 at the end? How did the backdoor command influence that?

In the example you cite, the AD would only be changing for the specific default route–not all routes.

What you are seeing is the effect of the backdoor command. The backdoor command changes the BGP administrative distance of the selected route from 20 to 200. 200 is chosen because that AD is worse than any IGP’s AD. This way, BGP will be used as a last resort if either static or other IGP routing mechanisms become unavailable.

Why would someone run OSPF between 2 different AS? Could you please provide me with practical scenarios? Because I don’t understand that when there is BGP to run between AS then why is that OSPF being used.


Hello Ravi

There are many situations in which it would be beneficial for you to run an IGP such as OSPF between AS’s. These include:

  1. Allowing multiple routing protocols to inject routes into the routing table provides backup routing capabilities if a routing protocol fails
  2. In order for BGP to function, and in order for it to be able to share routing between neighbours, the neighbours themselves must be reachable to each other. In order to achieve this, an IGP is often used.

I hope this has been helpful!


Hi Rene,

Can you please help to explain what is BGP backdoor routes. As the page only shows practical. Please explain the theory also.

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!