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Etherchannel and STP

Recently there was a debate among some folks about how STP is handled within an Etherchannel. I understand that once an Etherchannel is defined, regardless of PAGP or LACP, that the new port-channel is introduced into the spanning tree and the physical ports are removed. That makes sense. The question is, how are BPDUs passed across the port-channel? I found an article (below) that references that for 2900s and 3550s, it uses the default port. That makes sense. But I can't find any documentation for other switches. 3560s? 4k? 6k? Nexus? Is the same functionality applied across the product lines?

2900/2950/2550 section:

The default port is used to transmit       traffic, such as Spanning Tree Protocol (STP), multicasts, and       unknown unicasts. The default port can be identified from the       output of the command show etherchannel summary by a       notation of d.


Both are good points. I could see how you could approach it from either direction.  In our case we would prefer the  convergence over the choke point given the preferred sizing of the port  channel. Perhaps the correct approach is to oversize the port channel to  the point that the loss of an upstream switching card or a member link  or two doesn't create a choke point.  I hate to over commit ports so much but maybe that is just the cost of redundancy.

I appreciate the feedback.


It's a loaded question.

Do you use truly use LACP for thruput gains or link redundancy? In our case, my focus in the design is link redundancy. I always scale to N+1. So in the event of a cable failure (rare but it does happen) I don't sacrifice thruput. No choke point. I admit, it costs me on port capacity but in most environments that is an acceptable hit for the availability guarantee.

On the same note, one of our bigger debates going on internally is whether to move to 10G cards in some of our closets where we are up to 4 or 6 cables in a bundle. Cost vs Performance vs etc....

Frequent Contributor

On top of all this info - very useful btw, I have this scenario:

  - SW_A is connected through 10Gbps link to SW_B

  - new wire is to connect SW_A and SW_B

  - that wire is bundled solely into a port-channel, resulting in one port-channel with 10Gbps link


Which path will STP choose in regard to STP port cost?

Both Port-channel (1 link on 10Gbps) and old 10Gbps link will have same STP cost of 2?

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