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Portchannel between 3750 switches



Switch A connects to Swich B and C using port channel. I am going to bring down one link on each.

switch A is server farm , switch B and C are core 01 and 02 . and all are 3750 switches.

1. what will be the impact on the network in terms of spanning tree recalculation

2. what duration are we talking about until the spanning tree convergence happens?

3.I plan to shut down the link from CLI to bring down the links


Liam Kenneally


In regards to 1) There should be no spanning tree recaulations, that is the benefit of using portchannels with spanning tree. If you loose say 1 of 3 links in a portchannel the switch does not have to re-calculate as the interface is still up! the switch will only recalculate is that whole portchannel comes down.

2)There is no convergence time as per above. If the portchannel was to go down for some reason convergence time would be dependant on the type of spanning tree you are running on the devices.

3)I don't understand what you mean by question 3. Do you mean You will be SSH/Telnetting to the device hence will loose connection with the CLI when the links go down? Again if you are removing a single interface from a portchannel, as long as their is existing interfaces the portchannel will remain up.

EDIT:- I will simulate this in a lab environment and let you know the outcome. I cannof find any Cisco documentaion to backup the above. Additionally I know Spanning-tree will not re-converge if a link fails. However I DO NOT know if you can remove an interface from a portchannel without bringing down the portchannel first.

I will confirm as soon as I have lab'ed the above.


Disclaimer:- All Information provided is free of charge and ADVISORY, all information provided should be researched and tested in a LAB environment before implementing. Additionally full backups of all devices should be taken before implementing any procedure advised.

Kind Regards,


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