OSPF Load-sharing on Unequal Cost Link

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Oct 31st, 2007
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Hi All,

Problem statement:- One single customer has taken two service providers and using MPLS sham links on both. All CE routers are configured as Area0 on both the clouds.

Issue :- Customer wants the load-sharing on both the service providers between two sites.

Clarity on secenario:-

MPLS ISP-A = All sites are connected to MPLS PEs and are part of Area0.

MPLS ISP-B = All sites are connected to MPLS PEs and are again part of Area0.

Both ISPs have given Sham links to customer sothat customer can receive intra-area LSAs (IA) type.

Now, customer wants to do load-sharing on both service providers.

As per my understanding of OSPF design guide, the manual workaround on cost would never yield an optimal routed network.

Please suggest the other workaround to achieve the load-sharing.

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bvsnarayana03 Thu, 11/01/2007 - 01:09
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Why do you have that doubt?

If the bandwidths are different from 2 ISP's then manual configuration of cost on both links at both ends can give the desired results.

gaurav_thapar79 Thu, 11/01/2007 - 03:44
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i do not want to use manual cost on the link to achieve this. Please suggest some other workaround.

Kevin Dorrell Thu, 11/01/2007 - 05:31
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I would be interested to know why you reject that solution.

How about this one: terminate the OSPF at the border routers and run a seperate OSPF domain for your internal routing. Redistribute your provider's routes into the internal OSPF as E2 externals. These will carry the same cost throughout your internal OSPF, so all your routers will load-share the traffic between the two ASBRs.

You will have to think of solutions to other problems this will undoubtedly cause, like NAT, RPF checks, etc. etc, but you could consider it as a basis.

Kevin Dorrell


gaurav_thapar79 Thu, 11/01/2007 - 11:05
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Hi Kevin,

I am not at all giving any routes to the customer via PE.SP have created sham links which are acting as superbackbone and giving IA routes to customer 's router.

All CEs are part of Area0 and connected to PEs thru sham links.

I can not put another router as Area0 and then advertise my prefixes and learn rest as External Type 2.


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