Default route- multiple isp issue

Unanswered Question
Jun 12th, 2008

I have this scenario (on the drawing) set up in a lab environment. We have 2 core routers with customers connected. We have 2 border routers. when customer A hits Core router 1 and goes out its default gateway, if it has the preferred route going out it has no problem. if it finds that the preferred route is on the other side on border router 2- it gets sent back to core router 1 (the correct path). But that router's default route points it back to border 1- causing a loop. How can I fix this without having the core routes handle all the internet routes?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
n.nandrekar Thu, 06/12/2008 - 20:49


Can you please give the exact details in the question? I cant understand where the problem is due to the non-specific terminology.

In the topology, I see 3 routers marked as xxx-core-1 and 2 as xxx-border-1 . I dont see any core-2 or border-2 and neither do I see "customer A" Also which is the default gateway?



Harold Ritter Fri, 06/13/2008 - 06:54


This would be normal behavior if you are running iBGP between your border routers through the internal network and that internal network doesn't have the BGP routes.

To solve the issue, you would usually either run BGP on the internal routers, have a direct link between the border routers or not run iBGP between the border routers.


Harold Ritter Fri, 06/13/2008 - 09:28


The topology is exactly what I explained in my previous posting.

Solution: Do not run iBGP between the border routers, run iBGP on the core routers or have a direct link between the border routers.


n.nandrekar Fri, 06/13/2008 - 23:07

Hi Greg!

I guess you already have got the solution from Harold. The only way you can prevent the traffic from being sent back to the core router is if the border router doesnot know the other path out. This is possible if you dont run i-bgp between the border routers.

But using this method you would not be able to load balance correctly on the 2 links available. The traffic will take the path that the default route points to.

The other solution through which you can load balance, is that have a direct link between the border routers so that if the preffered path is through the other border router, the first one sends the traffic directly to the other border router through the direct link and would not send it to the core routers.

The third oprion is offcourse to run full mesh iBGP so that the core routers can make optimal routing decision.



gregwoodson Tue, 06/17/2008 - 07:29

We created a direct link using a GRE tunnel between the 2 routers. We created the bgp peer within the tunnel and it works smoothly. Thanks for the input!


This Discussion