understanding relationship between STP and loopback errors
I had a network topology described on "Spanning-tree on VLAN 1 is enabled" image. All the ports were in VLAN 1 and were sending keepalive messages with 10s interval. Then I executed "no spanning-tree vlan 1" on all four switches and all of the sudden some switch ports became err-disabled and "Keepailive packet loop-back detected" error messages were logged. How to explain this? For example how is it possible that SW1 port Fa0/19 received its keepalive frame?