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?

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xXErebuS

You turned spanning tree off in a looped network. SW1 fa0/19 sent out a loopback keepalive and b/c of the loop it received it back so it disabled the port. You can put "no keepalive" to test .... on the ports to turn off loopback detection and then you'll have a loop but the links should be up.

dmcneil330

Basically the switches are trying to save you from creating a broadcast storm in your network in VLAN 1. A nice feature for switches.