3Com Baseline 2928 PWR Plus User Manual page 214

Device
Port CP1 receives the configuration BPDU of Device A
{0, 0, 0, AP2}. Device C finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {2, 0, 2, CP1}, and updates the
configuration BPDU of CP1.
Port CP2 receives the configuration BPDU of port BP2 of
Device B {1, 0, 1, BP2} before the configuration BPDU is
updated. Device C finds that the received configuration
BPDU is superior to the configuration BPDU of the local
port {2, 0, 2, CP2}, and therefore updates the
configuration BPDU of CP2.
After comparison:
The configuration BPDU of CP1 is elected as the
optimum configuration BPDU, so CP1 is identified as the
root port, the configuration BPDUs of which will not be
changed.
Device C compares the calculated designated port
configuration BPDU {0, 10, 2, CP2} with the configuration
BPDU of CP2, and CP2 becomes the designated port,
and the configuration BPDU of this port will be replaced
with the calculated configuration BPDU.
Device C
Then, port CP2 receives the updated configuration
BPDU of Device B {0, 5, 1, BP2}. Because the received
configuration BPDU is superior to its own configuration
BPDU, Device C launches a BPDU update process.
At the same time, port CP1 receives periodic
configuration BPDUs from Device A. Device C does not
launch an update process after comparison.
After comparison:
Because the root path cost of CP2 (9) (root path cost of
the BPDU (5) plus path cost corresponding to CP2 (4)) is
smaller than the root path cost of CP1 (10) (root path cost
of the BPDU (0) + path cost corresponding to CP2 (10)),
the BPDU of CP2 is elected as the optimum BPDU, and
CP2 is elected as the root port, the messages of which
will not be changed.
After comparison between the configuration BPDU of
CP1 and the calculated designated port configuration
BPDU, port CP1 is blocked, with the configuration BPDU
of the port unchanged, and the port will not receive data
from Device A until a spanning tree calculation process is
triggered by a new event, for example, the link from
Device B to Device C going down.
After the comparison processes described in the table above, a spanning tree with Device A as the root
bridge is established as shown in
Comparison process
Figure 1-3.
1-7
BPDU of port after
comparison
CP1: {0, 0, 0, AP2}
CP2: {1, 0, 1, BP2}
Root port CP1:
{0, 0, 0, AP2}
Designated port CP2:
{0, 10, 2, CP2}
CP1: {0, 0, 0, AP2}
CP2: {0, 5, 1, BP2}
Blocked port CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}