1-7Device Comparison process BPDU of port aftercomparisonz Port BP1 receives the configuration BPDU of Device A {0, 0,0, AP1}. Device B finds that the received configuration BPDUis superior to the configuration BPDU of the local port {1, 0,1,BP1}, and updates the configuration BPDU of BP1.z Port BP2 receives the configuration BPDU of Device C {2, 0,2, CP2}. Device B finds that the configuration BPDU of thelocal port {1, 0, 1, BP2} is superior to the receivedconfiguration BPDU, and discards the received configurationBPDU.BP1: {0, 0, 0, AP1}BP2: {1, 0, 1, BP2}Device Bz Device B compares the configuration BPDUs of all its ports,and determines that the configuration BPDU of BP1 is theoptimum configuration BPDU. Then, it uses BP1 as the rootport, the configuration BPDUs of which will not be changed.z Based on the configuration BPDU of BP1 and the path cost ofthe root port (5), Device B calculates a designated portconfiguration BPDU for BP2 {0, 5, 1, BP2}.z Device B compares the calculated configuration BPDU {0, 5,1, BP2} with the configuration BPDU of BP2. If the calculatedBPDU is superior, BP2 will act as the designated port, andthe configuration BPDU on this port will be replaced with thecalculated configuration BPDU, which will be sent outperiodically.Root port BP1:{0, 0, 0, AP1}Designated portBP2:{0, 5, 1, BP2}z Port CP1 receives the configuration BPDU of Device A {0, 0,0, AP2}. Device C finds that the received configuration BPDUis superior to the configuration BPDU of the local port {2, 0, 2,CP1}, and updates the configuration BPDU of CP1.z Port CP2 receives the configuration BPDU of port BP2 ofDevice B {1, 0, 1, BP2} before the message was updated.Device C finds that the received configuration BPDU issuperior to the configuration BPDU of the local port {2, 0, 2,CP2}, and updates the configuration BPDU of CP2.CP1: {0, 0, 0, AP2}CP2: {1, 0, 1, BP2}By comparison:z The configuration BPDUs of CP1 is elected as the optimumconfiguration BPDU, so CP1 is identified as the root port, theconfiguration BPDUs of which will not be changed.z Device C compares the calculated designated portconfiguration BPDU {0, 10, 2, CP2} with the configurationBPDU of CP2, and CP2 becomes the designated port, andthe configuration BPDU of this port will be replaced with thecalculated configuration BPDU.Root port CP1:{0, 0, 0, AP2}Designated portCP2:{0, 10, 2, CP2}Device Cz Next, port CP2 receives the updated configuration BPDU ofDevice B {0, 5, 1, BP2}. Because the received configurationBPDU is superior to its old one, Device C launches a BPDUupdate process.z At the same time, port CP1 receives configuration BPDUsperiodically from Device A. Device C does not launch anupdate process after comparison.CP1: {0, 0, 0, AP2}CP2: {0, 5, 1, BP2}
