H3C S3100-52P Operation Manual page 252

Operation Manual – MSTP
H3C S3100-52P Ethernet Switch
Table 1-5 Comparison process and result on each device
Device
Device
A
Device
B
Comparison process
Port AP1 receives the configuration BPDU of
Device B {1, 0, 1, BP1}. Device A finds that the
configuration BPDU of the local port {0, 0, 0, AP1}
is superior to the configuration received message,
and discards the received configuration BPDU.
Port AP2 receives the configuration BPDU of
Device C {2, 0, 2, CP1}. Device A finds that the
BPDU of the local port {0, 0, 0, AP2} is superior to
the received configuration BPDU, and discards the
received configuration BPDU.
Device A finds that both the root bridge and
designated bridge in the configuration BPDUs of
all its ports are Device A itself, so it assumes itself
to be the root bridge. In this case, it does not make
any change to the configuration BPDU of each
port, and starts sending out configuration BPDUs
periodically.
Port BP1 receives the configuration BPDU of
Device A {0, 0, 0, AP1}. Device B finds that the
received configuration BPDU is superior to the
configuration BPDU of the local port {1, 0,1, BP1},
and updates the configuration BPDU of BP1.
Port BP2 receives the configuration BPDU of
Device C {2, 0, 2, CP2}. Device B finds that the
configuration BPDU of the local port {1, 0, 1, BP2}
is superior to the received configuration BPDU,
and discards the received configuration BPDU.
Device B compares the configuration BPDUs of all
its ports, and determines that the configuration
BPDU of BP1 is the optimum configuration BPDU.
Then, it uses BP1 as the root port, the
configuration BPDUs of which will not be changed.
Based on the configuration BPDU of BP1 and the
path cost of the root port (5), Device B calculates a
designated port configuration BPDU for BP2 {0, 5,
1, BP2}.
Device B compares the calculated configuration
BPDU {0, 5, 1, BP2} with the configuration BPDU
of BP2. If the calculated BPDU is superior, BP2 will
act as the designated port, and the configuration
BPDU on this port will be replaced with the
calculated configuration BPDU, which will be sent
out periodically.
1-8
Chapter 1 MSTP Configuration
BPDU of port
after
comparison
AP1: {0, 0, 0,
AP1}
AP2: {0, 0, 0,
AP2}
BP1: {0, 0, 0,
AP1}
BP2: {1, 0, 1,
BP2}
Root port BP1:
{0, 0, 0, AP1}
Designated port
BP2:
{0, 5, 1, BP2}