Possible loop?

[CODIT]

I have (2) 2960XR's (Switch B) connected with a stacking module.  They have port channels to (2) other 9500's (Switch A) in L3/L2 and another port channel to another (2) 2960XR's (Switch C) used as a access layer switch.  One of the stacking modules in Switch B failed and both switches assumed the master role.  Traffic through switch B was intermittent from switch A and C.  It seems there was a loop, but I'm not sure how I can prevent this.  We have spanning tree configured with loop guard.

[Jaderson Pessoa]

I have had the same situation two months ago.

I think that you have two possibility.
1. On the LAG, verify if the cables are plugged only one switch in stack.
2. Separe them from stack and run through mstp.

[Richard Burts]

If the stacking module failed then both switches in B are operating independently. And it certainly could create a loop. If we think about the individual members of B as B1 and B2 then think about this: A frame is received in B1 in vlan 2. It goes through the Etherchannel from B1 through switch A and is delivered to B2 and then is forwarded through the Etherchannel through switch C and delivered to B1 which detects the loop. As far as prevention is concerned I believe that your options are either to prevent the module failure/repair the module or to change the Etherchanels through A and C.

HTH

Rick