170 CHAPTER 10: RSTP CONFIGURATIONThe comparison process of each Switch is as follows.■ Switch A:AP1 receives the configuration BPDU from Switch B and finds out that the localconfiguration BPDU priority is higher than that of the received one, so itdiscards the received configuration BPDU. The configuration BPDU is processedon the AP2 in a similar way. Thus Switch A finds itself the root and designatedbridge in the configuration BPDU of every port; it regards itself as the root,retains the configuration BPDU of each port and transmits configuration BPDUto others regularly thereafter. By now, the configuration BPDUs of the twoports are as follows:Configuration BPDU of AP1: {0, 0, 0, AP1}.Configuration BPDU of AP2: {0, 0, 0, AP2}.■ Switch B:BP1 receives the configuration BPDU from Switch A and finds that the receivedBPDU has a higher priority than the local one, so it updates its configurationBPDU.BP2 receives the configuration BPDU from Switch C and finds that the localBPDU priority is higher than that of the received one, so it discards the receivedBPDU.By now the configuration BPDUs of each port are as follows: ConfigurationBPDU of BP1: {0, 0, 0, AP1}, Configuration BPDU of BP2: {1, 0, 1, BP2}.Switch B compares the configuration BPDUs of the ports and selects the BP1BPDU as the optimum one. Thus BP1 is elected as the root port and theconfiguration BPDUs of Switch B ports are updated as follows.The configuration BPDU of the root port BP1 retains as {0, 0, 0, BP1}. BP2updates root ID with that in the optimum configuration BPDU, the path cost toroot with 5, sets the designated bridge as the local Switch ID and thedesignated port ID as the local port ID. Thus the configuration BPDU becomes{0, 5, 1, BP2}.Then all the designated ports of Switch B transmit the configuration BPDUsregularly.■ Switch C:CP2 receives from the BP2 of Switch B the configuration BPDU {1, 0, 1, BP2}that has not been updated and then the updating process is launched. {1, 0, 1,BP2}.CP1 receives the configuration BPDU {0, 0, 0, AP2} from Switch A and SwitchC launches the updating. The configuration BPDU is updated as {0, 0, 0, AP2}.By comparison, CP1 configuration BPDU is elected as the optimum one. TheCP1 is thus specified as the root port with no modifications made on itsconfiguration BPDU. However, CP2 will be blocked and its BPDU also remainsthe same, but it will not receive the data (excluding the STP packet) forwardedfrom Switch B until spanning tree calculation is launched again by some newevents. For example, the link from Switch B to C is down or the port receives abetter configuration BPDU.
