69
Device
Comparison process
Configuration BPDU on
ports after comparison
22.
Updates its configuration BPDU.
Port C2 performs the following tasks:
23.
Receives the original configuration BPDU of Port B2 {1, 0,
1, Port B2}.
24.
Determines that the received configuration BPDU is
superior to the existing configuration BPDU {2, 0, 2, Port
C2}.
25.
Updates its configuration BPDU.
Device C performs the following tasks:
26.
Compares the configuration BPDUs of all its ports.
27.
Decides that the configuration BPDU of Port C1 is the
optimum.
28.
Selects Port C1 as the root port with the configuration
BPDU unchanged.
Based on the configuration BPDU and path cost of the root
port, Device C calculates the configuration BPDU of Port C2 {0,
10, 2, Port C2}. Device C compares it with the existing
configuration BPDU of Port C2 {1, 0, 1, Port B2}. Device C
determines that the calculated configuration BPDU is superior
to the existing one, selects Port C2 as the designated port, and
replaces the configuration BPDU of Port C2 with the calculated
one.
•
Root port (Port C1): {0,
0, 0, Port A2}
•
Designated port (Port
C2): {0, 10, 2, Port C2}
Port C2 performs the following tasks:
29.
Receives the updated configuration BPDU of Port B2 {0,
5, 1, Port B2}.
30.
Determines that the received configuration BPDU is
superior to its existing configuration BPDU {0, 10, 2, Port
C2}.
31.
Updates its configuration BPDU.
Port C1 performs the following tasks:
32.
Receives a periodic configuration BPDU {0, 0, 0, Port A2}
from Port A2.
33.
Determines that it is the same as the existing
configuration BPDU.
34.
Discards the received BPDU.
•
Port C1: {0, 0, 0, Port
A2}
•
Port C2: {0, 5, 1, Port
B2}
Device C determines that the root path cost of Port C1 (10)
(root path cost of the received configuration BPDU (0) plus
path cost of Port C1 (10)) is larger than that of Port C2 (9) (root
path cost of the received configuration BPDU (5) plus path cost
of Port C2 (4)). Device C determines that the configuration
BPDU of Port C2 is the optimum, and selects Port C2 as the
root port with the configuration BPDU unchanged.
Based on the configuration BPDU and path cost of the root
port, Device C performs the following tasks:
35.
Calculates a designated port configuration BPDU for Port
C1 {0, 9, 2, Port C1}.
36.
Compares it with the existing configuration BPDU of Port
C1 {0, 0, 0, Port A2}.
37.
Determines that the existing configuration BPDU is
superior to the calculated one and blocks Port C1 with the
configuration BPDU unchanged.
Port C1 does not forward data until a new event triggers a
spanning tree calculation process: for example, the link
between Device B and Device C is down.
•
Blocked port (Port C1):
{0, 0, 0, Port A2}
•
Root port (Port C2): {0,
5, 1, Port B2}
