H3C S5100-SI Series Operation Manual page 226

Operation Manual – MSTP
H3C S5100-SI/EI Series Ethernet Switches
Note:
Principle for configuration BPDU comparison:
The configuration BPDU that has the lowest root bridge ID has the highest priority.
If all configuration BPDUs have the same root bridge ID, they will be compared for
their root path costs. If the root path cost in a configuration BPDU plus the path cost
corresponding to this port is S, the configuration BPDU with the smallest S value
has the highest priority.
If all configuration BPDUs have the same root path cost, the following configuration
BPDU priority is compared sequentially: designated bridge IDs, designated port IDs,
and then the IDs of the ports on which the configuration BPDUs are received. The
switch with a higher priority is elected as the root bridge.
Selection of the root bridge
At network initialization, each STP-compliant device on the network assumes itself to
be the root bridge, with the root bridge ID being its own bridge ID. By exchanging
configuration BPDUs, the devices compare one another's root bridge ID. The device
with the smallest root bridge ID is elected as the root bridge.
Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Table 1-3 Selection of the root port and designated ports
Step
1
2
A non-root-bridge device takes the port on which the optimum
configuration BPDU was received as the root port.
Based on the configuration BPDU and the path cost of the root
port, the device calculates a designated port configuration BPDU
for each of the rest ports.
The root bridge ID is replaced with that of the configuration
BPDU of the root port.
The root path cost is replaced with that of the configuration
BPDU of the root port plus the path cost corresponding to the
root port.
The designated bridge ID is replaced with the ID of this device.
The designated port ID is replaced with the ID of this port.
1-5
Chapter 1 MSTP Configuration
Description