Defining the Spanning Tree Protocol – STP
Identifying traffic loops

A bridge loop is observed when a frame that is forwarded circulates, cyclically and redundantly; this occurs where there is no mechanism to manage the redundant Layer 2 paths.

Example: Flooded Unicast Frames and Bridge Loops
Station A has two potential paths to station B by way of the two intermediate bridges. The following describes what happens if station A sends to station B, if there were no provisions enabled to deal with redundant paths.

  • Station A transmits the frame destined for station B onto segment A. Both bridges on segment A pick up the frame on their bridge ports 1/1 and 2/1. Both bridges populate their respective MAC tables indicating that station A resides on segment A, on bridge ports 1/1 and 2/1.
  • Both bridges forward the frame to segment B. Station B receive the frame, and both bridges also see the same frame, with station A’s MAC address in the source address (SA) field, coming from the other bridge. The bridges will now incorrectly forward all frames for station A to segment B. When station B responds to station A, the frame will be dropped by both bridges because it will be received on the same bridge ports that it considers the destination of station A
  • If station A, or any station, sends a broadcast, the effects of the Layer 2 loop would be much worse. The destination MAC address would be FF-FF-FF-FF-FF-FF. This would cause each bridge to forward the frame out all bridge ports except the bridge port upon which the frame was received. The broadcast frame would also be forwarded to the originating bridge, which would again forward the same broadcast out all bridge ports. This broadcast would continue until the loop is shut down or until the bridge could no longer handle the load.