64Device Comparison processConfiguration BPDUon ports aftercomparisonplus path cost of Port C2 (4). Device C determines that theconfiguration BPDU of Port C2 is the optimum, and selectsPort C2 as the root port with the configuration BPDUunchanged.Based on the configuration BPDU and path cost of the rootport, Device C performs the following operations:37. Calculates a designated port configuration BPDU forPort C1 {0, 9, 2, Port C1}.38. Compares it with the existing configuration BPDU ofPort C1 {0, 0, 0, Port A2}.39. Determines that the existing configuration BPDU issuperior to the calculated one and blocks Port C1 withthe configuration BPDU unchanged.Port C1 does not forward data until a new event triggers aspanning tree calculation process: for example, the linkbetween Device B and Device C is down.After the comparison processes described in Table 6, a spanning tree with Device A as the rootbridge is established, as shown in Figure 20.Figure 20 The final calculated spanning treeThe configuration BPDU forwarding mechanism of STPThe configuration BPDUs of STP are forwarded according to these guidelines:• Upon network initiation, every device regards itself as the root bridge and generatesconfiguration BPDUs with itself as the root. Then it sends the configuration BPDUs at a regularhello interval.• If the root port receives a configuration BPDU superior to the configuration BPDU of the port,the device performs the following operations:{ Increases the message age carried in the configuration BPDU.{ Starts a timer to time the configuration BPDU.{ Sends this configuration BPDU through the designated port.• If a designated port receives a configuration BPDU with a lower priority than its configurationBPDU, the port immediately responds with its configuration BPDU.• If a path fails, the root port on this path no longer receives new configuration BPDUs and the oldconfiguration 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 newspanning tree calculation process to establish a new path to restore the network connectivity.
