Multiple MPLS Carriers/Networks

Unanswered Question
Sep 19th, 2007

Does anyone have any experience with using 2 MPLS carriers for load balancing. We'd like to have a primary MPLS network with ATT and then a backup MPLS network with MCI. But rather than use MCI strictly for backup we'd like to load balance traffic across both networks. Most of our bigger sites will have a connection into both clouds. Anyone have any idea how we'd handle the routing for that?

I have this problem too.
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swaroop.potdar Wed, 09/19/2007 - 12:44

Your edge CE should be connected to both the clouds. And then its matter of load balancing on 2 equal cost paths. So what ever protocol you use you will have to ensure that both paths are seen as equal cost/best paths.

Apart from that it will be load balanced across both the clouds on its own.

So in short its like have two logical connections to the whole of you network from each edge CE persepctive.



mheusing Wed, 09/19/2007 - 14:03


How to handle routing for CEs connecting to two MPLS providers for load balancing really depends on your topology and the PE-CE routing protocols.

Will one CE connect to both or are two separate CE routers in use? In the latter case, which routing protocol is used between them and the rest of your location internally?

Which CE-PE routing protocols will be used? The same everywhere, or different ones?

F.e. with one CE and different CE-PE protocols administrative distance will complicate load balancing (if not make it impossible).

The more dynamic protocols involved the more attention you need to avoid cross provider routing loops.

It would take too much time to cover all possible combinations, could you provide more info?

Regards, Martin

sonjam Fri, 09/21/2007 - 12:28

We plan to use BGP and have 2 CE devices (Cisco 3845). Initially we thought we'd have 1 CE connect to 1 MPLS Provider, but don't know how we would we load balance. From reading the this it's an interesting concept (and may be somewhat easier to config) to have 1 CE connect to both MPLS Providers. Thoughts?

Joseph W. Doherty Sat, 09/22/2007 - 14:11

I assist in supporting an environment which might almost be identical to that you want to move into.

The first issue you might bump into is the way EBGP will prefer to always use its CE-PE link if it sees otherwise equal paths. Simple solution, divide your traffic to your CEs. We do this either by using GLBP for small sites, or via equal cost paths within the LANs routing topology to the CE routers.

Second issue you might bump into is if the AS topologies are different between the MPLS providers making AS hop counts different between sites. We addressed this issue by defining and maintaining our own logical topology that looks identical between the different MPLS providers.

Both of the prior principally perform static load balancing but if you working with 12.4 or later, you might also want to look at OER. It can dynamically load balance, either with the static techniques, above, or without them.


We also deal with the situation where two sites are not on the same provider's MPLS. (Usually the result of some problem.) For this, we leak routes between the providers at major hubs permitting traffic to transit between the providers for sites that can not be reached on the same provider.

etienne.basset Thu, 09/20/2007 - 00:27


as a general rule i should avoid load-balancing between 2 given sites.

Site A should have one carrier as primary path to site B and the other as backup (and B to A should take the same carrier).

But site A to site C could take the other carrier as primary

Do not load-balance A to B traffic between the two carriers it will be an exploitation and troobleshooting nightmare ...

I don't know your PE/CE protocol, but you should definitively use BGP to have flexible policies


swaroop.potdar Sat, 09/22/2007 - 16:36

Whatever protocol you want to use as a PE-CE protocol or you want to use 1 or 2 CE's, you may want to stick to one thumb rule, dont make any site a transit for one MPLS SP. If you stick to this rule thing s hsould be pretty much fine and you would be able to achieve your objective positvely.

(Dont create a transit meaning, when you learn routes to site x through SP_1 on CE1 interface 1 dont advertise those routes out of CE2 [or CE1 interface 2 to SP_2]) And for the load balancing out of a site use the default CEF method of per destination throughout your each site. And send the routes down from your CE with equal cost/metric to the core LAN device in each site.

This way redundancy would be maintained for that site and you would have the needed load balancing as well.




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