LACP load balancing across multiple switches

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Sep 12th, 2007
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Hello all, after much google'ing and searching through this forum I must post.

I have somehow managed to load balance two ports on a 2950 to ports on two different 3560G switches. The result is what I wanted but how I achieved this I have no idea.

The attached image shows a screenshot of the current topology as seen from the Cisco Network Assistant. As you can see ClientSwitch1 (circled in red) has both links to ServerSwitch1 and ServerSwitch2 in FWD state. The other client switches each have 1 port forwarding and one blocking.

Keeping 1 port in BLK mode makes sense as STP will do this to prevent a loop. It's obvious though looking at ClientSwitch1 that a load balanced path to multiple switches is possible.

My question to the group is, "How can I accomplish what appears to be accomplished on ClientSwitch1? That is, what needs to be done to have the rest of the client switches have active/active paths to the two server layer switches?"

My CCNA expired years ago and now I am again one of those fools stumbling his way around while our organization is looking for a Sys-Admin to hire. Anyone interested in moving to Twin Falls, Idaho?


Abbie Mashaal

Database Administrator

Seastrom Manufacturing Co, Inc.

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glen.grant Wed, 09/12/2007 - 17:09
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Remember each vlan has its own spanning tree so its hard to tell anything from that picture . If you have say 5 vlans on that switch each vlan has its own spanning tree and somewhere will block each of those vlans "if" there is a built in loop in the design if not no ports will be blocked .It could be blocked on 1 side or the other depending on if you set your spanning tree roots or not . unless that link between the server switches is a trunk allowing all vlans then there really is no built in loop in the design . that being said i don't think a lot of us have used net assistant so i am speculating here.

seastrom-mfg Thu, 09/13/2007 - 06:28
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No VLANs are configured; everything is still on VLAN 1.

Interesting point though. I'm going to investigate whether trucking ports have anything to do with the perception of being up.

In general, does anyone know how to create multiple load balanced paths?



If you stick with layer-2 then the easiest way is to trunk some VLANs on one link and the other VLANs on the other link so that the traffic loads are shared.

You can set spanning tree to favor one over the other on a per-VLAN basis. That way if one link really fails all traffic will still be carried on the one that remains.


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