7-7
Bridge
Comparison process
BPDU of port after
comparison
z
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.
z
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.
CP1: {0, 0, 0, AP2}
CP2: {1, 0, 1, BP2}
After comparison:
z
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.
z
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.
Root port CP1:
{0, 0, 0, AP2}
Designated port CP2:
{0, 10, 2, CP2}
z
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.
z
At the same time, port CP1 receives periodic configuration
BPDUs from Device A. Device C does not launch an update
process after comparison.
CP1: {0, 0, 0, AP2}
CP2: {0, 5, 1, BP2}
Device C
After comparison:
z
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.
z
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.
Blocked port CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}
After the comparison processes described in the table above, a spanning tree with Device A as the root
bridge is established as shown in
.
