H3C Technologies H3C S7500E Series Switches - Manual (Page 160)

13-8
Device Comparison process
BPDU of port after
comparison
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.
CP1: {0, 0, 0, AP2}
CP2: {1, 0, 1, BP2}
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.
Root port CP1:
{0, 0, 0, AP2}
Designated port CP2:
{0, 10, 2, CP2}
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.
CP1: {0, 0, 0, AP2}
CP2: {0, 5, 1, BP2}
Device C
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.
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
Figure 13-3 .