3Com MSR 50 Series Configuration Manual page 446

446 C 23: MSTP C
HAPTER
n
ONFIGURATION
Upon initialization of a device, each port generates a BPDU with itself as the root
bridge, in which the root path cost is 0, designated bridge ID is the device ID, and
the designated port is the local port.
Selection of the optimum configuration BPDU
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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 11 Selection of the optimum configuration BPDU
Step Description
1 Upon receiving a configuration BPDU on a port, the device performs the
following processing:
■
■
2 The device compares the configuration BPDUs of all the ports and chooses
the optimum configuration BPDU.
Principle for configuration BPDU comparison
The configuration BPDU that has the lowest root bridge ID has the highest
■
priority.
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.
If all configuration BPDU have the same root path cost, they will be compared
■
for their designated bridge IDs, then their designated port IDs, and then the IDs
of the ports on which they are received. The smaller the ID, the higher message
priority.
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 device 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
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The process of selecting the root port and designated ports is as follows:
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.
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.