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244 CHAPTER 8: STP OPERATION
The comparison process of each switch is:
■ Switch A
GigabitEthernet1/1/1 receives the configuration BPDU from Switch B and finds
out that the local configuration BPDU priority is higher than that of the
received one, so it discards the received configuration BPDU.
The configuration BPDU is processed on the GigabitEthernet1/1/2 in a similar
way. Thus, Switch A finds itself the root and designated switch in the
configuration BPDU of every port; it regards itself as the root, retains the
configuration BPDU of each port and transmits configuration BPDU to others
regularly thereafter. By now, the configuration BPDUs of the two ports are as
follows:
Configuration BPDU of GigabitEthernet1/1/1: {0, 0, 0, e1/1/1}
Configuration BPDU of GigabitEthernet1/1/2: {0, 0, 0, e1/1/2}
■ Switch B
GigabitEthernet1/1/7 receives the configuration BPDU from Switch A and finds
that the received BPDU has a higher priority than the local one, so it updates its
configuration BPDU.
GigabitEthernet1/1/4 receives the configuration BPDU from Switch C and finds
that the local BPDU priority is higher than that of the received one, so it
discards the received BPDU.
By now the configuration BPDUs of each port are as follows:
Configuration BPDU of GigabitEthernet1/1/7: {0, 0, 0, e1/1/1}
Configuration BPDU of GigabitEthernet1/1/4: {1, 0, 1, e1/1/4}
Switch B compares the configuration BPDUs of the ports and selects the
GigabitEthernet1/1/7 BPDU as the optimum one. Thus, GigabitEthernet1/1/7 is
elected as the root port and the configuration BPDUs of Switch B ports are
updated as follows.
The configuration BPDU of the root port GigabitEthernet1/1/7 remains {0, 0, 0,
e1/1/1}. GigabitEthernet1/1/4 updates the root ID with the root ID in the
optimum configuration BPDU, updates the path cost to root with 5, sets the
designated switch as the local switch ID and the designated port ID as the local
port ID. Thus, the configuration BPDU becomes {0, 5, 1, e1/1/4}.
All the designated ports of Switch B then transmit the configuration BPDUs
regularly.
■ Switch C
GigabitEthernet1/1/1 receives from the GigabitEthernet1/1/4 of Switch B, the
configuration BPDU {1, 0, 1, e1/1/4} that has not been updated, then the
updating process is launched. {1, 0, 1, e1/1/4}.
GigabitEthernet1/1/5 receives the configuration BPDU {0, 0, 0, e1/1/2} from
Switch A, and Switch C launches the updating. The configuration BPDU is
updated as {0, 0, 0, e1/1/2}.
By comparison, the GigabitEthernet1/1/5 configuration BPDU is elected as the
optimum one. The GigabitEthernet1/1/5 is thus specified as the root port with
no modifications made on its configuration BPDU. However,
GigabitEthernet1/1/1 is blocked and its BPDU also remains the same, but it will
not receive the data (excluding the STP packet) forwarded from Switch B until