Port 3 (GigabitEthernet1/0/3) of VLAN0020 is alternate blocking
Port path cost 4, Port priority 128, Port Identifier 128.3.
Designated root has priority 32788, address 0018.ba87.c880
Designated bridge has priority 32788, address 0018.ba87.c880
Designated port id is 128.165, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point by default
BPDU: sent 1, received 168
After identifying the link with BPDU transmission and reception problems, isolate the problem further by checking whether any errors, etc. are counted on the interface transmitting/receiving BPDU, and whether there are any problems with the usage of resources such as CPU or with the traffic path in the device, etc.
In terms of preventing the STP loop in the event of any failure, UDLD and Loop guard are effective. You may want to consider using such functions as well.
Original Document: Cisco Support Community Japan DOC-29635