Loop Guard blocked a port: possible one-way trouble
%SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet1/0/49 on VLAN0101.
In plain English: Loop Guard is a spanning-tree safety net.
Platform: Cisco IOS / IOS XE. Values shown are the documented defaults — confirm them on your platform with the check commands below.
What it means
Loop Guard is a spanning-tree safety net. A port that was receiving the 'you are not the root bridge' messages (BPDUs) stopped hearing them. Normally that would tempt spanning tree to open the port and risk a loop; Loop Guard blocks it instead. Treat the message as a symptom: something between this switch and the rest of the network stopped delivering those messages — often the same one-way-link fault UDLD hunts.
Likely causes, in order
A one-way link: one fiber strand or one direction of a link has failed, so BPDUs only flow one way.
The neighboring switch is overloaded and stopped sending BPDUs on time.
A device was inserted between the switches that does not pass spanning-tree traffic (a cheap switch, a misconfigured bridge, a media converter).
Check, in order
show spanning-tree inconsistentportsEvery port currently held down by a spanning-tree safety feature.
show udld neighborsCross-check for a one-way link on the same port — the two watchdogs often catch the same fault.
show interfaces Gi1/0/49 counters errorsPhysical errors that explain why BPDUs stopped arriving.
The fix
Fix the physical path: clean or replace fiber, replace the failing optic, remove anything between the switches that eats BPDUs.
Confirm BPDUs are arriving again (the port leaves the inconsistentports list on its own).
Never respond by disabling Loop Guard — an unguarded version of this fault is a full network loop.
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