H3C S5120-EI Series - Page 107

1-6
Device Port name BPDU of port
CP1 {2, 0, 2, CP1}
Device C
CP2 {2, 0, 2, CP2}
z Comparison process and result on each device
The following table shows the comparison process and result on each device.
Table 1-5 Comparison process and result on each device
Device Comparison process
BPDU of port after
comparison
Device A
z Port AP1 receives the configuration BPDU of Device B
{1, 0, 1, BP1}. Device A finds that the configuration
BPDU of the local port {0, 0, 0, AP1} is superior to the
received configuration BPDU, and therefore discards the
received configuration BPDU.
z Port AP2 receives the configuration BPDU of Device C
{2, 0, 2, CP1}. Device A finds that the BPDU of the local
port {0, 0, 0, AP2} is superior to the received
configuration BPDU, and therefore discards the received
configuration BPDU.
z Device A finds that both the root bridge and designated
bridge in the configuration BPDUs of all its ports are
itself, so it assumes itself to be the root bridge. In this
case, it does not make any change to the configuration
BPDU of each port, and starts sending out configuration
BPDUs periodically.
AP1: {0, 0, 0, AP1}
AP2: {0, 0, 0, AP2}
z Port BP1 receives the configuration BPDU of Device A
{0, 0, 0, AP1}. Device B finds that the received
configuration BPDU is 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 the local port {1, 0, 1, BP2} is superior to the
received configuration BPDU, and therefore discards the
received configuration BPDU.
BP1: {0, 0, 0, AP1}
BP2: {1, 0, 1, BP2}
Device B
z Device B compares the configuration BPDUs of all its
ports, and determines that the configuration BPDU of
BP1 is the optimum configuration BPDU. Then, it uses
BP1 as the root port, the configuration BPDUs of which
will not be changed.
z Based on the configuration BPDU of BP1 and the path
cost of the root port (5), Device B calculates a designated
port configuration 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. As the
calculated BPDU is superior, BP2 will act as the
designated port, and the configuration BPDU on this port
will be replaced with the calculated configuration BPDU,
which will be sent out periodically.
Root port BP1:
{0, 0, 0, AP1}
Designated port BP2:
{0, 5, 1, BP2}