H3C S3100 Series Operation Manual page 227

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 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.
By comparison:
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.
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.
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 {0, 10, 2, CP2}, Device C
launches a BPDU update process.
Device C
At the same time, port CP1 receives configuration BPDUs
periodically from Device A. Device C does not launch an
update process after comparison.
By comparing the configuration BPDU {0, 0, 0, AP2} of port
CP1 and the configuration BPDU {0, 5, 1, BP2} of port CP2,
the device selects the root port:
Because the two configuration BPDUs carry the same root
bridge ID, the root path costs are further compared. 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
the optimum BPDU, and CP2 is elected the root port, the
configuration BPDUs {0, 5, 1, BP2} of which will not be
changed.
The device determines the role of port CP1:
The configuration BPDU {0, 0, 0, AP2} of CP1 is compared
with the calculated designated port configuration BPDU {0, 9,
2, CP1}. Because the former is superior to the latter, 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.
After the comparison processes described in the table above, a spanning tree with Device A as the root
bridge is stabilized, as shown in
Comparison process
Figure 1-3.
1-8
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 CP1:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}