Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member


STP Fails

I had some problems in my network recently.

My network has about fifteen (15) Access Layer Switchs where are connected my users. Each Access Switch is with redundant to each Core Layer Switch (actually two Switchs) and I also have three (03) distribution layer Switchs where are my servers (about 300 servers). All the switchs are pluged in My Core Layer with redundant.

Example of One of the Access layers Switchs

Access Layer Switch1 Port 1/1 -- Core Layer Switch-A Port 3/6

Access Layer Switch1 Port 1/2 -- Core Layer Switch-B Port 3/6

I have "BPDU Skewing" activated in my network and I got some messages. The logs showing that BPDU was coming with delay:

Dec 16 13:01:58 %SPANTREE-2-BPDU_SKEWING: BPDU skewed with a delay of 10 secs (max_age/2)

Last week my network went down because both ports on

Access Layer Switch1 changed to Forwarding Mode.

My network come back to operate after reboot

all distribution Layer Switch where are pluged my server ( My big boss did it the reboot :-(

I don't undertansd what happened with the STP

to casue LOOP in my network.

Is possible both ports on Catalyst to stay in Forwarding mode at te same time ?

Does anyone have any explanation ??


Re: STP FAILS - Causing LOOP

One possible explanation is that you had a unidirectional link or the BPDU's are discarded due to CRC errors or so. When there are no more BPDU's coming in, blocked interfaces will go into forwarding mode. When the link is only bad in one way you can still get a loop.