STP Loops

[bmack2121]

We had a trunk link go down on a switch due to the fiber cord being bent. I replaced the cord and wanted to add a redundant link just in case this happens again. When I added the second fiber to trunk link, a loop occured instantly. Once it was unplugged everything worked again. How do I fix this ?

[Mark Malone · Accepted solution]

before plugging in the link set the stp cost on each interface on the backup link to 2500 this will put it staright into blocking mode as the primary will have a cost of 4 or 19 depending on port type

Instead though i would build an etherchannel as you will get redundency and more bandwidth that way rather than having links which are shut down

[bmack2121]

That was quick. Thanks Mark.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If you have STP active, it should have blocked one of the trunk links.  If you don't have a STP active, you should have.

To avoid the need for STP to block the additional link, if your devices support it, Etherchannel the two ports into one logical.

[bmack2121]

Thanks Joe.