1-7Device Comparison process BPDU of port aftercomparisonz Port CP1 receives the configuration BPDU of Device A{0, 0, 0, AP2}. Device C finds that the receivedconfiguration BPDU is superior to the configurationBPDU of the local port {2, 0, 2, CP1}, and updates theconfiguration BPDU of CP1.z Port CP2 receives the configuration BPDU of port BP2 ofDevice B {1, 0, 1, BP2} before the configuration BPDU isupdated. Device C finds that the received configurationBPDU is superior to the configuration BPDU of the localport {2, 0, 2, CP2}, and therefore updates theconfiguration BPDU of CP2.CP1: {0, 0, 0, AP2}CP2: {1, 0, 1, BP2}After comparison:z The configuration BPDU of CP1 is elected as theoptimum configuration BPDU, so CP1 is identified as theroot port, the configuration BPDUs of which will not bechanged.z Device C compares the calculated designated portconfiguration BPDU {0, 10, 2, CP2} with the configurationBPDU of CP2, and CP2 becomes the designated port,and the configuration BPDU of this port will be replacedwith the calculated configuration BPDU.Root port CP1:{0, 0, 0, AP2}Designated port CP2:{0, 10, 2, CP2}z Then, port CP2 receives the updated configurationBPDU of Device B {0, 5, 1, BP2}. Because the receivedconfiguration BPDU is superior to its own configurationBPDU, Device C launches a BPDU update process.z At the same time, port CP1 receives periodicconfiguration BPDUs from Device A. Device C does notlaunch an update process after comparison.CP1: {0, 0, 0, AP2}CP2: {0, 5, 1, BP2}Device CAfter comparison:z Because the root path cost of CP2 (9) (root path cost ofthe BPDU (5) plus path cost corresponding to CP2 (4)) issmaller than the root path cost of CP1 (10) (root path costof the BPDU (0) + path cost corresponding to CP2 (10)),the BPDU of CP2 is elected as the optimum BPDU, andCP2 is elected as the root port, the messages of whichwill not be changed.z After comparison between the configuration BPDU ofCP1 and the calculated designated port configurationBPDU, port CP1 is blocked, with the configuration BPDUof the port unchanged, and the port will not receive datafrom Device A until a spanning tree calculation process istriggered by a new event, for example, the link fromDevice B to Device C going down.Blocked port CP2:{0, 0, 0, AP2}Root port CP2:{0, 5, 1, BP2}After the comparison processes described in the table above, a spanning tree with Device A as the rootbridge is established as shown in Figure 1-3.
