STP Loop Guard

The STP loop guard feature provides additional protection against Layer 2 forwarding


 loops (STP loops). An STP loop is created when an STP blocking port in a redundant 


topology erroneously transitions to the forwarding state. This usually happens because 


one of the ports of a physically redundant topology (not necessarily the STP blocking port) 


no longer receives STP BPDUs. In its operation, STP relies on continuous reception or 


transmission of BPDUs based on the port role. The designated port transmits BPDUs, and 


the non-designated port receives BPDUs.


When one of the ports in a physically redundant topology no longer receives BPDUs, the 


STP conceives that the topology is loop free. Eventually, the blocking port from the 


alternate or backup port becomes designated and moves to a forwarding state. This 


situation creates a loop.


The loop guard feature makes additional checks. If BPDUs are not received on a non-


designated port, and loop guard is enabled, that port is moved into the STP loop-




inconsistent blocking state, instead of the listening / learning / forwarding state. Without 


the loop guard feature, the port assumes the designated port role. The port moves to the 


STP forwarding state and creates a loop.


When the loop guard blocks an inconsistent port, this message is logged:

• CatOS
  

  %SPANTREE-2-LOOPGUARDBLOCK: No BPDUs were received on port 3/2 in vlan 3. Moved to
   loop-inconsistent state.

• Cisco IOS
  

  %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port FastEthernet0/24 on