Identifying a Bridging Loop

Suspect a loop if you see the following:

  • You capture traffic on a link and see the same frames multiple times.
  • All users in a bridging domain have connectivity problems at the same time.
  • There is abnormally high port utilization.

To remedy a loop quickly, shut redundant ports and then enable them one at a time. Some switches enable debugging of STP to help in diagnosing problems. The following commands are useful for isolating a bridging loop


Spanning-Tree Best Practices

To optimize data flow in the network, design and configure Spanning Tree in the following ways:

  • Statically configure switches to be the primary and secondary root bridges by setting priority values.
  • Consider which interfaces will become designated and root ports (possibly set port priorities/path cost).
  • Tune STP using the tools detailed in this section.
  • Enable UDLD aggressive mode on all fiber interfaces.
  • Design STP domains that are as simple and contained as possible by using multilayer switches and routed links.
  • Use PVRST+ or MST for the fastest convergence times.