load balancing accross two T1 to different ISP

Unanswered Question
Mar 4th, 2008

My company is going to get a second T1 from another ISP which will use all 24 channels for data. The old T1 we will keep only 8 channels for data. The rest will be voiced.

What is the best way to do some traffic balancing between two of T1 so the outgoing traffic will be in a ratio 3:1 (24 channels new T1: 8 channels old T1) ?

We do not need load balancing based on the traffic type - but all the traffic in general.

Also, I am not quite sure how our second ISP will handle this but we need also the incoming traffic to reach us through two routes and the new route of the new T1 to have a better metric so approximately 3/4 of the traffic comes from that line.

What should the ISP do to accomplish this? The problem is that they are not very commmunicative and I want to make sure they are doing it the right way.

Any suggestion, link to some documentation would be very very welcome.

The router we have is Cisco 2610 IOS 12.3

IP/Firewall/IDS package.

Thank you very much

I have this problem too.
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Paolo Bevilacqua Tue, 03/04/2008 - 09:41


in summary, is not that you can do much. If the router does nat, you can do some kind of load balancing having two static default routes, or specific ones, using route-map in the nat statements.

Incoming traffic will reach you via the two routes but only on the addresses specific to each provider, that is you cannot have traffic destined to one ISP address go over the othe r link and vice-versa. That would reguire BGP that is probably our of question for your situation.

Also IOS would need upgrade but the router you have is slow can have trouble in keeping up.

gentianhila Tue, 03/04/2008 - 12:20

I actually was thinking to do it in another way since it is too complicated.

For the incoming traffic: I will move some of the DNS records to point to the new network so it will take maybe more than half of the traffic routed that way.

For the outgoing traffic I will use EIGRP load balancing in my router. We will upgrade to a new router soon.

Is this something that can work?

Paolo Bevilacqua Tue, 03/04/2008 - 13:13

No it won't work. EIGRP in an interior routing protocol, and is not use with isp.

Beside, you have the fundamental problem of address. Eash set of address must stick to the link of the isp that gave to you. As consequence, router must do nat usng these addresses. This way, you can have some kind of balancing and redundacy of the two routes.

gentianhila Tue, 03/04/2008 - 14:13

So there is no way to share the load of outbound traffic only? For the inbound I can use DNS and revert most of the traffic but still I woud like to have some load balancing for the outbound traffic.

I have heard about per packet balancing but that affects video quality.

So there is no hope at all for this situation? Would some other equipment help on this?

Joseph W. Doherty Tue, 03/04/2008 - 10:54

One technique that might work, if you're using default routes outbound, and if you can have more than one point to the same next hop, point three at the faster link, one at the slower link.

Assuming you have different public addresses from each provider, the foregoing would also split the return traffic into the same ratio.

Any traffic inbound, that isn't a response to outbound traffic, you might be able to control with addresses returned via DNS. (Don't recall whether DNS will allow multiple references of the same name to the same address.)

gentianhila Tue, 03/04/2008 - 13:39

Right now I use a static route. And I will have different ip addresses from each provider.

Can you elaborate a little bit more on what you mean when you say: "point three at the faster link, one at the slower link.".

Can you have more than one default route? That is what you're saying?

Thank you very much.

Joseph W. Doherty Tue, 03/04/2008 - 13:46

I may be mistaken, but I thought it's possible to have multiple statics point to the same next hop, including the default. I'm suggesting something like:

ip route x.x.x.x

ip route x.x.x.x

ip route x.x.x.x

ip route y.y.y.y

At first glance, the above seems pointless, but what I recall it does is round robins between the four default routes.

Paolo Bevilacqua Tue, 03/04/2008 - 13:51

Joseph, if entered as above, the first three commands will collapse into one and become the same to all effects.

Joseph W. Doherty Tue, 03/04/2008 - 14:14

Ah, sorry about that. Wasn't sure, and at the moment, don't have access to any Cisco routers so I could have tried it before suggesting it.

Still have some dim memory about using multiple statics for load balancing. If I find anything more about it, I'll post another note.


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