BGP Backdoor Routes


(Rene Molenaar) #1

This topic is to discuss the following lesson:


(mukesh c) #2

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. .


(Rene Molenaar) #3

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


(Eng_ A) #4

Great topic. Thanks Rene


(Marty S) #5

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?


(Rene Molenaar) #6

Hi Marty,

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

MPLS L3 VPN Configuration

Rene


(Mahmoud H) #7

Hi Rene,
when we write

router bgp 100
network 0.0.0.0 mask 0.0.0.0 backdoor

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


(Rafa) #8

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


(Andrew P) #9

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


(Andrew P) #10

Rafa,
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.


(Ravi P) #11

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.

Thanks,
Ravi


(Lazaros Agapides) #12

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!

Laz