Dual isp's link

Unanswered Question
Mar 4th, 2009


We wud be having a setup of 2 isp links terminating on a 6500 at a remote site.its an mpls link.I need to run a dynamic routing protocol preferably ospf in the internal network on 6500 and have traffic loadshared between both CE devices connected to the 6500.

Pls suggest me something with configs.


I have this problem too.
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Giuseppe Larosa Wed, 03/04/2009 - 06:42

Hello Sunny,

I would connect the two CE to the C6500 using two point-to-point routed links.

c6500 - CE1 net

c6500 - CE2 net

under ospf

router ospf 10

network area 0

network area 0

+ other commands to advertise site's subnets

CE1 and CE2 have to inject routes with similar characteristics:

same prefix len

same ospf route type (O IA or O E1 or O E2)

same metric

Their configuration depend on the type of service you got:

if it is a leased line service you can run OSPF everywhere

the same can happen if it is some form of L2 VPN service (EoMPLS or VPLS)

if the service is L3 VPN the configuration depends on the PE-CE routing protocol in use: it can be OSPF, EIGRP, RIP or BGP.

Typically BGP can be the preferred solution (by the provider)

This should provide the load balancing in both directions.

it would be effective in presence of multiple traffic flows (traffic variety)

Hope to help


suthomas1 Thu, 03/05/2009 - 00:12

Thanks, Lets say if they run BGP on the PE-CE segment, would they be running ibpg on the CE-CE segment to have failure redundancy.Basically, do i need to tell providers anything specific to configure on their end so that this works smooth & would they/me need to run any redistribution on either side.

& are the ones above the only config that i will have to do apart from advertising my internal n.w thru ospf to reach the other site & how do i verify if the links are loadbalanced after configuration.

Thanks in advance!

Giuseppe Larosa Thu, 03/05/2009 - 03:16

Hello Sunny,

with eBGP sessions on the PE-CE links you need to advertise local subnets.

I would suggest to use network commands for this.

if the two PE providers belong to two different providers each one has to perform its own routing in his VPN context.

about the IBGP session :

I wouldn't use it if the site is simple.

BGP routes coming from other sites have to be redistributed into OSPF with similar parameters as I've explained in my first post.

if an eBGP session fails the involved CE is going to remove its own advertisements in OSPF.

Hope to help


suthomas1 Thu, 03/05/2009 - 04:28

The PE-CE is managed by the provider, only L3 switch is in our control.Nonethless i can ask the provider to tread on particular lines with valid logics, if any.

So the local subnet advertisement from my L3 wud be done by the provider,as i see from your post.The providers are same.

About the ibgp between the CE's, i had this query as say in case one of the ebgp sessions goes off, does that mean the existing connectivity/session thru that pair would have to go all the way to cloud and den start back again thru the other PE-CE pair?

Giuseppe Larosa Thu, 03/05/2009 - 07:02

Hello Sunny,

>> in case one of the ebgp sessions goes off

the other PE-CE will be the only exit/ingress point for traffic no need for unnecessary travel on the cloud

the PE of the broken session will stop to advertise your site IP subnets

Hope to help



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