Which is better, L2 or L3 Etherchannel Connections?

rclaxton1
rclaxton1 used Ask the Experts™
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If uplinking two core IOS catalyst devices together which would be better to setup, a layer 3 or layer 2 etherchannel for load balancing between them? From the responses I'll then ask more detail questions about them. Thanks.
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Well; it's not really a matter one being better than the other. The etherchannel is a physical binding of several links together. You can then configure that group of links as either a layer 2 or a layer 3 interface. The load balancing is handled differently on each type but that is just because they are passing a different type of traffic.

 Currently do you use a layer 2 or a layer 3 link to connect these devices? You should use whatever type of link is appropriate for your situation. If you need to pass a trunk over this link make it layer 2. If you need to connect a router to another router use layer 3.

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Commented:
Ok, I believe you mean a logical binding of physical ports for aggregated bandwidth. I believe the default agorithm for layer 2 and layer 3 is XOR for the load-balancing meaning souce,destination via mac (L2) and source destination via IP (L3). So if two routers (6509s) that have one standard gig trunked uplink between them using both L2 and L3 which would be more effecient; building another uplink of the L2 architecture and binding the two or creating a seperate L3 etherchannel between the routers and keep the standard trunked uplink between them for L2 only.
Commented:
This is how I would do it. First I would do the design without etherchannel.  In other words, don't think of the connection between the 6509s as an etherchannel. Think of the connection between the 6509s as a plain old single cable.

Now, decide how you want to do the routing, addressing, and vlans. Do you want the link between the 6509s to be a /31 with ip addresses on both sides, and then route from one 6509 to the other? Or do you want a vlan to span both 6509s and put the link in that vlan?  Or do you want multiple vlans to span both 6509s and trunk the vlans over the link?

The answer depends on the desired arrangements of vlans and routing, which should be dictated by the applications and/or network hierarchy.

After you have your decision, convert the single link to an etherchannel and you are done. Load-balancing will be taken care of by the etherchannel hashing algorithm on the interface. If you have lots of tcp flows, the load-balancing will be even across the members in the bundle.

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Commented:
Excellent response Donlee thanks. There're may instances to think of when designing it and you hit it right on! In my case since multiple vlans are spanning the 6509s I went with the Layer2 build and is working great. Thanks again. --Rob.

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