two routers and two isp

Unanswered Question
Apr 8th, 2009

Hey guys,

I'm sure this will be easy for you but for some reason I can't get my head around it.

I have two routers routerA with Cable modem and routerb with t1 link

I'd like to configure it so that if the link to the t1 goes down traffic will go out the DSL

This sounds easy but I'm having a brain fart

Please help


I have this problem too.
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lamav Wed, 04/08/2009 - 15:26

Hi there:

I am going to assume that you are running BGP with your 2 ISPs.

I am also going to assume that you want a hot/standby and not a load sharing scenario.

I'll call the router connected to the primary ISP R1 and the secondary to the fallback ISP connection R2.

The primary ISP is A and the secondary is B.

R1 and R2 can accept defaults from their respective ISPs.

You can have R1 and R2 in an HSRP configuration and have the backend routers point to their HSRP VIP for a route to the Internet.

R1 will be the primary and can use interface tracking within HSRP to decrement its priority in the event that it suffers a WAN link failure to ISP A. In that case, R2 will become the primary router for Internet access.

This will establish a deterministic scenario for outbound traffic.

To ensure that inbound traffic from the Internet uses your primary ISP, ISP A, and primary router, R1, to get back to your enterprise network, you can have R2 advertise an extended AS path to ISP B, thereby making it very unattractive to ISP B and its peers, forcing them instead to go through ISP A.

By the way, it's not easy. There's a lot of things to take into consideration, so don't beat yourself up. :-)



lamav Wed, 04/08/2009 - 15:53

By the way, I neglected to mention that you would run iBGP between R1 and R2 so that they can learn of each others defaults as a back up, especially for R1.

Joseph W. Doherty Wed, 04/08/2009 - 17:38

There are different ways to do this. How does traffic get initially directed to routers A and B? What's the routing configuration for them? Also, what are the platforms and installed IOS?


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