5.2 STP Processes
5.2.7 Topology changes and STP
As previously shown, STP implements a series of timers to prevent bridging loops from occurring within the network. It can also take 30-50 seconds for a network to converge to a new topology. While the network is converging, physical addresses that can no longer be reached are still listed in the switch table. Because these addresses are in the table, the switch will attempt to forward frames to devices it cannot reach. Fortunately, the STP change process requires the switch to clear the table faster in order to get rid of unreachable physical addresses.

The Figure illustrates a link failure between Switches D and E, which in turn triggers a topology change condition. This topology change condition triggers a topology change BPDU to be generated toward the root bridge. In order for this BPDU to reach the root bridge, each switch forwards the update out the root port to the designated bridge.

A topology change in a Spanning-Tree network triggers the following steps:

  • A bridge (Switch D) notices that a change has occurred to a link.
  • The bridge (Switch D) sends a topology change BPDU out the root port destined ultimately for the root bridge. The topology-change BPDU is indicated by a change to the 1-byte flag of the BPDU. The bridge will send out the topology change BPDU until the designated bridge for that segment responds with a topology-change acknowledgement (also in the 1-byte flag field).
  • The designated bridge (Switch B) for that segment sends out a topology-change acknowledgement to the originating bridge (Switch D). The designated bridge for that segment also sends a topology-change BPDU out its root port destined for either the designated bridge of the other segment or the root bridge.
  • When the root bridge receives the topology-change message, the root bridge will change its configuration message to indicate that a topology change is occurring. The root bridge sets the topology change in the configuration for a period of time equal to the sum of the FwdDelay and MaxAge parameters.
  • A bridge receiving the topology-change configuration message from the root bridge uses the FwdDelay timer to age out entries in the address table. This allows the device to age out entries faster than the normal 5-minute default so that stations that are no longer available will be aged out faster. The bridge continues this process until it no longer receives topology-change configuration messages from the root bridge.