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

Spantree Loop Guard (question)

Hi All,

I would like to ask about one feature:-

(Spantree loopguard default), what does this feature provide?? And when should I use it??


Re: Spantree Loop Guard (question)

Re: Spantree Loop Guard (question)


Ive never seen it configured in the networks I've worked on, but loopguard is STP feature that will monitor ports that were once receiving BPDU packtes. If these port stop receiving it, this could create a loop, so if you have loopguard enabled on this port the STP will block the port for going to FWD state.



The loop guard is intended to provide additional protection against L2 forwarding loops (STP loops). An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to forwarding state. This usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) stopped receiving STP BPDUs. In its operation, STP relies on continuous reception or transmission of BPDUs, depending on the port role (designated port transmits, non-designated port receives BPDUs).

When one of the ports in a physically redundant topology stops receiving BPDUs, the STP conceives the topology as loop free. Eventually, the blocking port from the alternate or backup port becomes designated, and moves to forwarding state, thus creating a loop.

With the loop guard, an additional check is made. If BPDUs are not received any more on a non-designated port and the loop guard is enabled, that port will be moved into the STP loop-inconsistent blocking state instead of moving to the listening / learning / forwarding state. Without the loop guard, the port would assume the designated port role. The port would move to STP forwarding state, and thus create a loop. "


So, on which ports should the loop guard be enabled? The most obvious answer is on the blocking ports. However, this is not totally correct. The loop guard needs to be enabled on the non-designated ports (more precisely, on root and alternate ports) for all possible combinations of active topologies."


if it does, I'd appreciate if you rated this post.


Re: Spantree Loop Guard (question)

Hi Vlad,

thanks for your input, its really helpfull.

thank you,

CreatePlease to create content