3Com 2928 User Guide - Page 215

Figure 1-3 The final calculated spanning tree Device A With priority 0 AP1 5 BP1 BP2 Device B With priority 1 AP2 4 CP2 Device C With priority 2 The spanning tree calculation process in this example is only a simplified process. The BPDU forwarding mechanism in STP z Upon network initiation, every device regards itself as the root bridge, generates configuration BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval. z If it is the root port that received a configuration BPDU and the received configuration BPDU is superior to the configuration BPDU of the port, the device increases the message age carried in the configuration BPDU following a certain rule and starts a timer to time the configuration BPDU while sending out this configuration BPDU through the designated port. z If the configuration BPDU received on a designated port has a lower priority than the configuration BPDU of the local port, the port immediately sends out its own configuration BPDU in response. z If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUs and the old configuration BPDUs will be discarded due to timeout. In this case, the device will generate configuration BPDUs with itself as the root. This triggers a new spanning tree calculation process to establish a new path to restore the network connectivity. However, the newly calculated configuration BPDU will not be propagated throughout the network immediately, so the old root ports and designated ports that have not detected the topology change continue forwarding data along the old path. If the new root ports and designated ports begin to forward data as soon as they are elected, a temporary loop may occur. STP timers STP calculation involves three important timing parameters: forward delay, hello time, and max age. z Forward delay is the delay time for device state transition. A path failure can cause spanning tree re-calculation to adapt the spanning tree structure to the change. However, the resulting new configuration BPDU cannot propagate throughout the network immediately. If the newly elected root ports and designated ports start to forward data right away, a temporary loop is likely to occur. 1-8