LACP/Etherchannel and broadcast storms

Unanswered Question
Dec 10th, 2009

Hi Gurus

Can anybody clarify the following questions, If w:e enable LACP or Etherchannel between 2 switches (A and B) composed of 4 links (in trunk). If switch A receive a broadcast

  1. Will it send it by the logical port or each individual port? I assume it will send a copy through the logical port (meaining LACP will select 1 physical port according to its algorithm and send the copy throught it).
  2. When switch B receives the broadcast through the selected physical port (according to the algorithm), will it be intelligent enough to undertand it came from 1 of the ports pertenining the LACP or Etherchannel and will not send a copy of the broadcast back to switch A (sort of split horizon mechanism)?

I might be facing a situation where I will need to shut down spanning-tree in specific vlan that will be part of the trunk in LACP mode and would liket to confirm how LACP behaves when dealing with broadcasts.

Thanks, Jorge

I have this problem too.
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Giuseppe Larosa Thu, 12/10/2009 - 13:03

Hello Jorge,

disabling STP for a specific vlan is not a good idea.

Are you running out of STP instances on one switch?

if so I would limit the number of vlans permitted on the bundle and other possible uplinks using switchport trunk allowd vlan.

LACP builds a bundle that acts as a single logical link, so you can have problems only if there are other links carrying that vlan out of the bundle.

Hope to help



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