20-8
Device
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 message was updated.
Device C finds that the received configuration BPDU is
superior to the configuration BPDU of the local port {2, 0, 2,
CP2}, and updates the configuration BPDU of CP2.
CP1: {0, 0, 0, AP2}
CP2: {1, 0, 1, BP2}
By comparison:
z
The configuration BPDUs 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
Next, port CP2 receives the updated configuration BPDU of
Device B {0, 5, 1, BP2}. Because the received configuration
BPDU is superior to its old one, Device C launches a BPDU
update process.
z
At the same time, port CP1 receives configuration BPDUs
periodically 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
By comparison:
z
Because the root path cost of CP2 (9) (root path cost of the
BPDU (5) + 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
remaining unchanged, and the port will not receive data from
Device A until a spanning tree calculation process is
triggered by a new condition, for example, the link from
Device B to Device C becomes 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 stabilized, as shown in
Figure 20-3
.
