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Community Member

STP Etherchannel Issue

I have an odd STP issue with a 5000, 5002 and a 3550 in the following simple configuration:

5000 ----------- 3550

1/1-2 0/1-2

isl trunk isl trunk

100/full 100/full

channel1 port-channel 1

6.3(7) 12.1(9)EA1c


These 2 switches are trunked thorugh a 2 port FEC...

A 5002 is redundantly connected to both the 5000 + 3550 with 2 trunks:

1/1 to 5/1 of the 5000 1/2 to 0/3 of the 3550

trunk isl 100/full trunk isl 100/full

The 5002 is using a SupII with 5.5(16)

The 5000 has been set to be the root bridge

The issue is, that ALL ports are forwarding in the STP configuration on all switches... the Po1 between the 5000 + 3550, the 5/1 + 1/1 between the 5000 + 5002, and 0/3 + 1/2 between the 3550 + 5002. For simplicity, VLAN 1 is the only VLAN being used and STP is enabled for VLAN 1 on all switches.

If I remove the FEC between the 5000 + 3550 and leave 2 redundant trunks, everything runs fine with STP and redundant ports that you would expect to be blocked/forwarding according to priority and costs are functioning as you would expect...

I have searched through numerous reference books that I own and have searched this board + TAC and I get no hints why STP fails when the FEC is configured...

I appreciate any help you can offer me!

Thanks for your time...

Community Member

Re: STP Etherchannel Issue


There are some known STP issues with 3550s, see bug-id CSCdy21905.

The problems should be solved in release 12.1(11)EA1 or later.


Community Member

Re: STP Etherchannel Issue

Thank you! That did the trick!

For now on, I will check know bugs before endlessly + needlessly searching for configuration errors...

Thanks again,


Cisco Employee

Re: STP Etherchannel Issue

There is a field notice for this issue

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