Operation Manual – MSTP
H3C S3100 Series Ethernet Switches
Chapter 1 MSTP Configuration
1-7
Table 1-5
Comparison process and result on each device
Device
Comparison process
BPDU of
port after
comparis
on
Device
A
z
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.
z
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.
z
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.
AP1: {0, 0,
0, AP1}
AP2: {0, 0,
0, AP2}
z
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.
z
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.
BP1: {0, 0,
0, AP1}
BP2: {1, 0,
1, BP2}
Device
B
z
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.
z
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}.
z
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.
Root port
BP1:
{0, 0, 0,
AP1}
Designate
d port
BP2:
{0, 5, 1,
BP2}
