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Multiple Instance Spanning-Tree Operation
STP Loop Guard
Figure 3-37. Example of Show Loop-Protect Output for VLAN Mode
STP Loop Guard
Spanning Tree (STP) is used to ensure a loop-free topology over the LAN.
Occasionally a hardware or software failure can cause STP to fail, creating
STP/forwarding loops that can cause network failures where unidirectional
links are used. The non-designated port transitions in a faulty manner because
the port is no longer receiving STP BPDUs.
STP Loop Guard causes the non-designated port to go into the STP loop-
inconsistent state instead of the forwarding state. In the loop-inconsistent
state the port prevents data traffic and BPDU transmission through the link,
therefore avoiding the loop creation. When BPDUs again are received on the
inconsistent port, it resumes normal STP operation automatically.
HP Switch(config)# show loop-protect 1-2
Status and Counters - Loop Protection Information
Transmit Interval (sec) : 5
Port Disable Timer (sec) : 5
Loop Detected Trap : Enabled
Loop Protect Mode : Vlan
Loop Protect Enabled VLANs : 20,30
Loop Loop Detected Loop Time Since Rx Port
Port Protect Detected on VLAN Count Last Loop Action Status
---- ------- -------- --------- -------- ----------- ---------- ---------
1 Yes Yes 20 1 45s send-disable Down
2 Yes No 0 send-disable Up
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