I have 4GBIC ports in access(3750) and core switches (6509)
What is better?
1) Running 4-port Etherchannel from access switch to a core switch, or
2) 2-port Etherchannel in 2 groups (One group in forwarding mode, and the other group is in blocking mode because of STP.
I think the first option is better because of the better throughput, and also redundancy since traffic can be re-routed to the remaining link if one of the link fails in the group. If (1) is the better design, then why would somebody go for the (2) option.
For the most part the first option is better as you have the added bandwidth of the extra links in addition to the redundancy that is inherent to Etherchannel.
With the second option, you essentially waste two links, since the blocked links aren't passing any traffic unless there is a failure on the forwarding etherchannel. Even then, once that link fails, the blocking link will have to go through the STP states before forwarding, which can take up to 50 seconds, whereas a single failed link in an Etherchannel will give you a near-instantaneous failover time.
Now, one scenario where one might choose option 2 is in a situation where you want to load balance specific VLANs over the redundant links. In this scenario, the two etherchannels would be trunks where the two links are fowarding for some VLANs and blocking for others and vice versa.
Thanks for your great answer. One thing in order to load balance between VLAN, then I will have to have two different VLAN in the same IDF. Won't I? I don't see any other ways to load balance between to EtherChannels Groups with just ONE VLAN.
If you just have one VLAN, then that makes the idea of VLAN load balancing moot. That feature is for when you have muliple VLANs on your switches. So in your case, with just one VLAN, the 4 port Etherchannel is the best way to go.
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