Operation Manual – MSTP
H3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration
1-4
z Selection of the optimum configuration BPDU
Each device sends out its configuration BPDU and receives configuration BPDUs from
other devices.
The process of selecting the optimum configuration BPDU is as follows:
Table 1-2 Selection of the optimum configuration BPDU
Step Description
1
Upon receiving a configuration BPDU on a port, the device performs
the following processing:
z If the received configuration BPDU has a lower priority than that of
the configuration BPDU generated by the port, the device will
discard the received configuration BPDU without doing any
processing on the configuration BPDU of this port.
z If the received configuration BPDU has a higher priority than that of
the configuration BPDU generated by the port, the device will
replace the content of the configuration BPDU generated by the
port with the content of the received configuration BPDU.
2
The device compares the configuration BPDUs of all the ports and
chooses the optimum configuration BPDU.
 Note:
Principle for configuration BPDU comparison:
z The configuration BPDU that has the lowest root bridge ID has the highest priority.
z If all the 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.
z 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.
z 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.
z Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
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