After the connection between two switches could not be established I went to see the log of the uplink switch and the first thing I have noticed was this: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk FastEthernet0/1 VLAN300. […]