Mar 30 14:11:44.325 aedt: %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk GigabitEthernet0/1 VLAN1.
Sw20 Gi0/1 (the one logging the above message) is connecting to Sw12 Gi0/2 which does not log any message.
I have checked a million times Sw20 Gi0/1 and Sw12 Gi0/2 and bot are access ports.
In fact the config under these interfaces are as simple as follow:
Did show cdp neigbor to make sure that Sw20 Gi0/1 goes to Sw12 Gi0/2
sw20#show cdp nei
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID Local Intrfce Holdtme Capability Platform Port ID
sw12 Gig 0/1 148 S I WS-C2950G- Gig 0/2