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Spanning Tree didn't stop a loop

 
thrtnastrx2
Occasional Advisor

Spanning Tree didn't stop a loop

A loop was created betweeen switch A and switch B and this caused the entire netowrk to go down.  Both switches had STP enabled and switch A was the root.  Switch A and switch B were connected via one copper port on each switch.  What I found was that switch B was configured with the interconnect port in a trunk, and switch A was not.  

example

  • switch A:
  • vlan 1
  • untagged 48

 

  • switch B:
  • trunk trk1 eth 48
  • vlan 1
  • untagged trk1

the loop was introduced between switch A and B on some other ports, say 5 and 6 which are configured as access ports but the loop was not detected.  Anyone know why this misconfiguration between the interconnect ports would cause this failure?