Load Balancing in OSPF

Answered Question
Oct 1st, 2009

RtrA is connected to RtrB with a T3 and T1 curcuit. Is there a way to load balancing between the two links? Any technotes would be greatly appreciated.

I have this problem too.
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Correct Answer by Richard Burts about 7 years 3 months ago

David

Like Giuseppe and Joseph I would advocate that you not configure to make both interfaces equal cost to OSPF. One of my reasons is that is you do configure this, then OSPF will attempt to treat both links equally. The mismatch in bandwidth capability is likely to result in overutilization of the low bandwidth link (with congestion and loss of packets) while the higher bandwidth link will probably be underutilized.

If you want both links to be used I agree that the Policy Based Routing appraoch should achieve what you want.

HTH

Rick

Correct Answer by Joseph W. Doherty about 7 years 3 months ago

Like Giuseppe, I would advise against using equal cost load balancing with a T3 and T1.

OSPF only supports equal cost load balancing, but EIGRP supports unequal cost load balancing.

However, even if you deal with the issues of trying to use EIGRP, both routing protocols perform flow based load balancing. I.e., they just round-robin new flows without consideration for how bandwidth intensive each flow is or load on the link. Perhaps an even better solution, if your platform supports it, would be usage of OER/PfR. OER/PfR can balance both links to the same load factor. E.g. Both could be at 20%.

The major issue with using OER/PfR, except for the latest PfR PIRO variant, only static route or BGP injection is supported.

Correct Answer by Giuseppe Larosa about 7 years 3 months ago

Hello David,

given the big speed mismatch I wouldn't make the two links to look like to have equal vost.

The way to do that has been explained by Edison.

Rather I would use PBR on both sides to send over the T1 selected flows (for example VoIP flows just to make an example)

Hope to help

Giuseppe

Correct Answer by Edison Ortiz about 7 years 3 months ago

You can manually cost the links to be equal at both routers' interfaces.

At the T3 serial interface you can apply ip ospf cost 1 and do the same on the T1 serial interface.

Once you do that, you will see 2 equal paths towards the destination.

__

Edison.

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Correct Answer
Edison Ortiz Thu, 10/01/2009 - 20:01

You can manually cost the links to be equal at both routers' interfaces.

At the T3 serial interface you can apply ip ospf cost 1 and do the same on the T1 serial interface.

Once you do that, you will see 2 equal paths towards the destination.

__

Edison.

Correct Answer
Giuseppe Larosa Thu, 10/01/2009 - 21:46

Hello David,

given the big speed mismatch I wouldn't make the two links to look like to have equal vost.

The way to do that has been explained by Edison.

Rather I would use PBR on both sides to send over the T1 selected flows (for example VoIP flows just to make an example)

Hope to help

Giuseppe

Correct Answer
Joseph W. Doherty Fri, 10/02/2009 - 04:54

Like Giuseppe, I would advise against using equal cost load balancing with a T3 and T1.

OSPF only supports equal cost load balancing, but EIGRP supports unequal cost load balancing.

However, even if you deal with the issues of trying to use EIGRP, both routing protocols perform flow based load balancing. I.e., they just round-robin new flows without consideration for how bandwidth intensive each flow is or load on the link. Perhaps an even better solution, if your platform supports it, would be usage of OER/PfR. OER/PfR can balance both links to the same load factor. E.g. Both could be at 20%.

The major issue with using OER/PfR, except for the latest PfR PIRO variant, only static route or BGP injection is supported.

Correct Answer
Richard Burts Sat, 10/03/2009 - 15:00

David

Like Giuseppe and Joseph I would advocate that you not configure to make both interfaces equal cost to OSPF. One of my reasons is that is you do configure this, then OSPF will attempt to treat both links equally. The mismatch in bandwidth capability is likely to result in overutilization of the low bandwidth link (with congestion and loss of packets) while the higher bandwidth link will probably be underutilized.

If you want both links to be used I agree that the Policy Based Routing appraoch should achieve what you want.

HTH

Rick

davidhuynh5 Sun, 10/04/2009 - 15:44

Thanks everyone. Not only does this forum answer the questions, but I appreciate the clear explanations.

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