67Figure 21 The final calculated spanning treeThe configuration BPDU forwarding mechanism of STPThe configuration BPDUs of STP are forwarded following these guidelines:• Upon network initiation, every device regards itself as the root bridge, generates configurationBPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.• If the root port received a configuration BPDU and the received configuration BPDU is superior tothe configuration BPDU of the port, the device increases the message age carried in theconfiguration BPDU following a certain rule and starts a timer to time the configuration BPDU whilesending this configuration BPDU through the designated port.• If the configuration BPDU received on a designated port has a lower priority than the configurationBPDU of the local port, the port immediately sends its own configuration BPDU in response.• If a path becomes faulty, the root port on this path no longer receives new configuration BPDUs andthe old configuration BPDUs will be discarded due to timeout. The device generates a configurationBPDU with itself as the root and sends the BPDUs and TCN BPDUs. This triggers a new spanningtree calculation process to establish a new path to restore the network connectivity.However, the newly calculated configuration BPDU cannot be propagated throughout the networkimmediately, so the old root ports and designated ports that have not detected the topology changecontinue forwarding data along the old path. If the new root ports and designated ports begin toforward data as soon as they are elected, a temporary loop might occur.STP timersThe most important timing parameters in STP calculation are forward delay, hello time, and max age.• Forward delayForward delay is the delay time for port state transition.A path failure can cause spanning tree re-calculation to adapt the spanning tree structure to thechange. However, the resulting new configuration BPDU cannot propagate throughout thenetwork immediately. If the newly elected root ports and designated ports start to forward dataimmediately, a temporary loop will likely occur.For this reason, as a mechanism for state transition in STP, the newly elected root ports ordesignated ports require twice the forward delay time before they transit to the forwarding state tomake sure that the new configuration BPDU has propagated throughout the network.• Hello timeThe device sends hello packets at the hello time interval to the neighboring devices to make surethat the paths are fault-free.• Max ageAB CRoot portDesignated portRoot bridgeNormal linkBlocked linkBlocked port
