load balancing on a router?

Unanswered Question
Jan 29th, 2009

Hi! It is possible to make load balancing of two different T1's on a single router?

I have this problem too.
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Richard Burts Thu, 01/29/2009 - 19:26


I wonder if there is some context to your question which is not obvious to us. But on the surface this is a very easy question. If a router is connected on two T1s, and if those T1s terminate on the same remote router, and if there are routes pointing at both T1s (might be static routes or might be a dynamic routing protocol) then yes the router will default to load balancing over those T1s.

Is there some other aspect that you are trying to get at?



robert.rosa Thu, 01/29/2009 - 19:40

Ok. We have two T1's (one from Company X and other from Company Y, both with different IP address). the two are connected to a single router (DSU/CSU). It it possible to make a load balancing of the T1's?

Thanks for your help

Richard Burts Thu, 01/29/2009 - 19:51


Perhaps we need a little more information about your environment to be able to give good answers to your question.

It does not matter much if the T1s are from different providers. And it does not matter that they have different IP addresses.

You tell us that both T1s connect to your local router. Do both T1s terminate on the same remote router? If so load balancing is easy. You would set up a static route with the first T1 as its next hop. Then you set up a similar static route with the second T1 as its next hop. When you do this the router will automatically load balance over both T1s.



robert.rosa Fri, 01/30/2009 - 06:18

Hi Rick,

Please look at the drawing. Thanks for your help.

any recommendations for a cisco router model that I should buy for this case?

Rick Morris Fri, 01/30/2009 - 11:04

To do what you want you will need to run BGP and announce your IP block to each provider. This will give you want you want.

What Rick was saying is that you would need the remote end(provider) to have both T-1's terminate in the same router. The only balancing you will get by static routes is outbound only.

Richard Burts Fri, 01/30/2009 - 11:52


For hardware you might be able to use an 1841 with 2 serial cards but I would be more comfortable recommending a 2800 (probably a 2811 or perhaps the 2821) which is clearly positioned for multiple T1s. (the 2811 would have 2 10/100 FastEthernet interfaces while the 2821 LAN interfaces are capable of 10/100/1000).

Rick makes an interesting point about load balancing for outbound being different than load balancing for inbound. Load balancing gets more complicated when the T1s terminate on different routers, and even more complex when they are to different providers. Depending on where you obtain your address space, running BGP to both providers might work, but if your address space is obtained from one of the providers then advertising that address space to the other provider may be difficult.

I implemented a project for a customer which might provide a model for something that you might consider. Like you, they have an internal network, traffic passes through a firewall to get to the outside router. The outside router connects to 2 different providers (the connections are not T1s but that does not matter). We set it up that most of their traffic goes to one provider and responses come back on that link while the outside router does Policy Based Routing and sends web traffic through the link to the other provider and gets responses for web traffic. Perhaps that might be an approach that you might consider.




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