CEF per packet load balacing

Unanswered Question
May 4th, 2009

Hello folks -

I've a Cisco 3750 stack that needs to be load balance traffic over two Cisco 2821 routers. The routers are connected on a VLAN on the 3750. These routers are going to be primarily used for communication between two Exchange Servers (one local, one across the WAN).

As a result, per destination load balancing that is enabled by default with CEF will not work will in this case. I am trying to enable per packet load balancing on the 3750 but it wouldn't give me an option. I have tried both under the global config mode and interface mode.

Any ideas on how I can do per packet load balacing between the 3750 and the two 2821 routers?

Thanks for your help!

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Paolo Bevilacqua Mon, 05/04/2009 - 15:54

You cannot do per-packet on the 3750, and you should not on the 2821.

Keep the default, observe interface utilization, come back with the results.

ksarin123_2 Mon, 05/04/2009 - 16:10

Can you explain why you recommend not doing per packet load balancing on the 2821's?

Joseph W. Doherty Mon, 05/04/2009 - 16:00

Unclear how per packet load balancing with assist you if destination is two different Exchange servers. However, are the 2821 ports defined as switched ports or routed ports? If switched, are the two 2821s on the same VLAN?

ksarin123_2 Mon, 05/04/2009 - 16:08

The destination is not two different Exchange Servers. One Exchange server is local to the site (where I have the 3750 and the 2821's) and one Exchange Server is across the WAN. These two Exchange servers need to talk to each other for the migration of the mailboxes.

The two 2821's are on switched ports and yes, they are in the same VLAN.

Paolo Bevilacqua Mon, 05/04/2009 - 16:13

Do not assume you will have no load balancing before observing results with the real application.

ksarin123_2 Mon, 05/04/2009 - 16:16

OK, so if the communication is always going to be between a single source-single destination, how will load balancing occur with per-destination load balancing method that is enabled by default with CEF?

I am still missing your point..

Paolo Bevilacqua Tue, 05/05/2009 - 09:08

My point is: in networking, before making assumptions, observe in depth the actual problem or behavior. Have you done that already ?

Joseph W. Doherty Mon, 05/04/2009 - 16:50

Ah, understand.

I had asked about switched vs. routed because I thought it might have some impact on CEF load sharing, but I just found in the latest 3750 reference guide, 12.2(50)SE, ip load-sharing [per-packet] is documented as an unsupported interface command.

If the 3750 is incapable, you could do it on a 2821. For instance, send all flows to prefered 2821 and from it have it "see" 2nd (probably best as equal cost) path via secondary 2821.

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