Showing results for 
Search instead for 
Did you mean: 

Transient Spanning-tree loop


We are a small service provider that uses Multiple Spanning-tree protocol throughout our Backhaul network. We also use Radio devices for transmission of data along with Layer2 switches(such as 6832x )

All the radios are transparent and act as Layer 1 devices.

Our layer 2 network consist of more than 10 loops which are managed by MST, and we devised myriad fail-over links which are forwards or blocked respectively based on spanning-tree decision. Here is when it gets tricky, whenever one of our radios which connects to a switch malfunctions, if that port is blocked before this problem happens, for a few seconds that port state, changes to forwarding mode which causes a transient loop throughout our network. Has anyone experienced such a problem and knows how to deal with it ? I'm looking for a loop preventaiton mechanism , would appreciate if you help us.

Rob Burchard
Cisco Employee

Hi ,


Based on your description of the issue, the loop guard stp enhancement may be what you are looking for to enhance your mst design. See below for supporting documentation.





Thanks Rob for your kind response.

Unfortunately loopguard doesn't work for us.

Consider this scenario :


All the sites between Sw1 and Sw2 act as layer one , and radio is the transmission.

Now imagine Sw1 that is connected to site1 port is in Alternate mode and Sw2 that is connected to site3 is in Designated mode. If we activate loopguard on Sw1 port  which is connected to site1, and Sw2 connections to site3 fails , we desire that Sw1 port change its state from Alternate to Forward mode , which is not going to happen if we activate loopgaurd on this port and this port goes to inconsistent mode in case of failure. If we had only two switch directly connected two each other , loopguard would be an elegant option , while in our network , there are numerous layer1 radios between switches.