Dual T1 loadbalancing

[mike.bradley]

I have 2 T1 circuits going to the same ISP. I would like traffic to be load balanced over both.

I have a bgp session with this ISP with the next hop being a loopback beyond the far end interfaces of my circuits.

Would ip load-sharing per-packet on both interfaces be the key?

[Georg Pauwen]

Hello Mike,

check this document, it has configuration examples for your purpose:

Load Sharing with BGP in Single and Multihomed Environments: Sample Configurations

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml

HTH,

GP

[thisisshanky]

You should be using ebgp multihop to peer properly with the ISP. You can use CEF with ip load-sharing per-packet for per packet load balancing, but you should be careful with using this command, as packets could come out of order (especially for applicatoin like VoIP, this can be critical). You could ask the ISP to bundle up the links using Multilink.

[ggatten]

Just curious why BGP is needed (or wanted) with only one ISP? The only case I can think of where this would make sense is if the circuits are actually POPped to different locations - especially different cities. If multi circuits to same ISP AND same location, I'd get rid of BGP - no need. Then you could do equal cost static default routes on both ends of the circuits. Other options mentioned already: CEF and MLPPP. Yet another might be IMA. All depends on what your ISP supports and what you're trying to accomplish. Might want to look into some QoS stuff too - as even two T1 can be easily saturated.

HTH - Good Luck!

Gary

[mike.bradley]

Thanks for the feedback. I didn't mention it in post but we do have two ISP's. Dual T1's to AT&T & MCI. We have ebgp multihop in place with both ISP's. The traffic is roughly split between the two ISP's. But looking at the individual lines going to each ISP I noted that the the AT&T lines are balanced and MCI are not. A large FTP job with MCI will pick a path over one T1 and saturate it.

I'll research CEF and talk to MCI about our options.

Thanks Again

Mike

[Richard Burts]

Depending on how often it happens, and on how concerned you are about it, you might consider configuration of policy routing to send ftp traffic over one link and some other grouping of traffic over the other link to balance it.

Or a solution that I might prefer would be to configure some QOS on the serials to allow the ftp but to assign more priority to the other traffic.

And I would be concerned about trying to do packet by packet load sharing. As has been pointed out the potential for out of order packets can be problematic. And I think out of order may be especially likely when the traffic is a mix of some maximum sized packets (ftp) and much smaller packets (other applications).

Having said that I will also say that I would approach with great caution attempts to manage and fine tune the bandwidth usage. Micromanagement of these issues takes lots of effort and the results are not always predictable. So how much of a problem is it really that a large ftp will saturate one link while it is running? And how much effort is it worth to address this issue?

HTH

Rick

[ggatten]

(4) total T1's then?

Agree some sort of QoS and maybe set a max bandwidth on ftp - so it can't saturate anything. The more problematic is the http downloads - seperating those from other http traffic. May also want to check out NBAR and LFI.

I'm not sure if your trying to LB / Bond the T1's so you can get a true 3Mb, or if you're concerned about LB between the ISP's - so you can get 6Mb/s? If the answer is LB between ISP's - balancing and policing egress traffic is fairly easy. The problem is in most environments the majority of traffic is ingress - which is far more difficult to control. Traffic coming in to you is controlled by how your routes are advertised all through the internet. Even if you try to manipulate your advertisements using the various BGP mechanisms, it's still a very diffult and time consuming process. Fail-over is easy - true load balancing or anything close is a challenge.

IMO, the bottom line is you can only support what your ISP supports. As you can see there are several options but your ISP may only support 1 or 2 - so no sense wasting time with the others.

I'd be curious what route you go with. It's been several years since I dealt with multi T1's, but I clearly remember what a PITA it was. Lots of time for minimal gains. Learned a lot though ;-)

Gary

[ggatten]

I think I understand now - you're more concerned with LB the T1's to each ISP than LB the traffic between ISP's, right? The bottom line is you want 3Mb/s pipes to each ISP?

Check out this doc:

http://www.cisco.com/en/US/products/hw/modules/ps2033/products_white_paper09186a0080091d4b.shtml

It discusses the most common methods I know most ISP's support - CEF, MLPPP, and IMA. IMA will require different WIC's and reprovisioning of your circuits, so may not be the best option for you now. IMO, MLPPP would be the way to go - IF your ISP's support it...

HTH

Gary

[mike.bradley]

Thanks all. Gary you described what I was looking for clearer then I me. The info has been helpful. I'm working with my ISP to get CEF with per-packet configured.