Unequal cost outbound load sharing with BGP

Unanswered Question
Oct 27th, 2009


I have a configuration that I'm desperately hoping that somebody can help me with.

I have a very large managed WAN from a telco where BGP is used as the sole routing algorithm. At the moment all sites have 2x connections a 10Mb/s primary link and a 2Mb/s backup link. There is currently no load sharing but I am being asked to make use of this 2Mb/s link to take the strain off the 10Mb/s link.

On each site we have a voice and a data VLAN. The voice traffic is about 1Mb/s and so would fit nicely on the 2Mb/s link.

Now I know that there are various options open to us to ensure that all inbound VOIP traffic is received on the 2Mb/s link, we are currently thinking of using MED to make the 2Mb/s link preferred for Voip.

My puzzle is how can I ensure that ALL outbound Voip traffic uses the 2Mb/s link. For various reasons the destination of VOIP traffic could be almost any IP address and so I am not able to use MED in a similar manner.

What I basically have to do here is to choose the outbound route based on the source subnet, rather than the destination, to ensure that all VOIP uses the 2Mb/s link and all data 10Mb/s link.

By doing this I will ensure that the traffic is completely symmetric.

Could somebody tell me what my options are for this kind of routing and point me to some links please?


I have this problem too.
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Giuseppe Larosa Tue, 10/27/2009 - 07:42

Hello James,

>> What I basically have to do here is to choose the outbound route based on the source subnet, rather than the destination, to ensure that all VOIP uses the 2Mb/s link and all data 10Mb/s link.

BGP is not enough you need to use PBR to be able to perform source based routing.

PBR uses route-maps that invoke ACLs to define traffic to be diverted



VoIP traffic could be confined to specific subnets for on-net calls.

The question are the off-net calls when the RTP stream from the IP Phone has to go to a voice gateway.

Hope to help


hamiltonjames17 Tue, 10/27/2009 - 09:04

Thanks Guiseppe,

I didn't mention PBR in my first post as I didn't want to lead any responses. Thank you for confirming my suspicions but unfortunately I've a feeling that there will be a reluctance to implement PBR.

I guess we could do something without PBR for on-net calls (and perhaps even the stream to the voice gateway) using the destination. As long as the VOIP VLANs are consistently addressed e,g, 10.x.2.y where x is a site and the third octet indicates voip we could perhaps setup a local preference for these subnets so that the router prefer the route advertised over the 2Mb/s link.

A quick follow on question, how much of a problem do you think asymetric voip traffic might be?

i.e. it would be simply for voip to always come in on the 2Mb/s and it would be easy enough to configure it so that on the outbound roughly half of it would route down the 10Mb/s and half down the 2Mb/s. I think that this might be okay if both links were 10Mb/s but I'm quite nervous with the big difference in bandwidth. We could end up with a voip call leaving site A on the 2Mb/s link, coming into site B on the 2Mb/s link, then leaving on the 10Mb/s link at B and back into A on its 2Mb/s link.

This sounds like a it might create problems. Would you agree?

Giuseppe Larosa Tue, 10/27/2009 - 09:34

Hello James,

depending on your address plan there is still one dimension to use:

prefix length:

if you are able to advertise more specific paths only on the 2 Mbps mesh of links: (example /24)

and to advertise less specific routes (example /23) on the 10 Mbps links you should be fine without PBR for on-net calls.

The same idea could be used for voice gateways ip addresses you could also advertise some specific /32 host routes in BGP only on the 2Mbps links sessions.

This requires some effort but it can be done.

you will need to use IP prefix-lists invoked in a route-map to be able to send /24 on one session and /23 on the other session.

Also, the ability to confine VoIP traffic on one type of link is important for the QoS aspects.

However, if one 2 Mbps link fails you need to provide routing failover over the 10 Mbps link (so the /23 prefix to be sent) and you need to provide resources for Voip to be carried inside an LLQ on the 10 Mbps link.

Hope to help



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