Load balancing between two ISP

Unanswered Question
Feb 13th, 2009

Pondering if it is a good idea to load balance between two ISP for our internet connection. We will be using bgp with default route and utilizing our own IP range.

Both ISP connections land into a single router and behind it is a ASA. All inside traffic is pointing to the ASA which nats all outgoing traffic to the interface which is our registered IP address.

Need advice/opinion on pro's and Cons.

Thank you very much


I have this problem too.
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Mohamad Qayoom Fri, 02/13/2009 - 14:19

We are currently load-balancing between two ISPs. If you are using BGP, you can use AS path prepending to force traffic to use a particular ISP.

Please let me know if you have any other questions.

Paolo Bevilacqua Fri, 02/13/2009 - 14:20

There are really no cons. If can you obtain your own address space and ASN, that is always the best choice.

syedsadiq Sun, 02/15/2009 - 04:38

By using AS PATH prepending and local preference we can loadshare Incoming traffic that is traffic from Internet to Corporate office but how will be the traffic flow of outgoing traffic from Corporate to Internet.

Mohamad Qayoom Mon, 02/16/2009 - 06:16

I don't think that there is an easy way to loadshare outbound traffic. Maybe you can use PBR(policy based routing) to achieve that task for some specific destinations.

Joseph W. Doherty Mon, 02/16/2009 - 09:44

The advantage of load balancing, you should have more bandwidth to work with which may improve performance. The disadvantage, depending on approach, extra complexity.

A simple approach is using two default routes, outbound, which will staticly split your traffic 50/50. Inbound, it's more complex because much depends on Internet topology and what your ISPs are doing.

A very powerful approach for outbound, is usage of OER/PfR, if your Cisco platform supports it. Either can dynamically load balance link utilization and can also monitoring actual end-to-end performance. PfR also has the option of informing your ISP how to process inbound traffic, but they would need to cooperate with the technology (although they don't have to run it).


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