3Com Switch 4800G 24-Port Configuration Manual page 200

200 C 23: MSTP C
HAPTER
n
ONFIGURATION
Designated port ID (in the form of port name)
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1 Specific calculation process of the STP algorithm
Initial state
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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 BPDUs and receives configuration BPDUs
from other devices.
The process of selecting the optimum configuration BPDU is as follows:
Table 38 Selection of the optimum configuration BPDU
Step Description
1 Upon receiving a configuration BPDU on a port, the device performs the following
processing:
If the received configuration BPDU has a lower priority than that of the
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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
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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.
Principle for configuration BPDU comparison
The configuration BPDU that has the lowest root bridge ID has the highest
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priority.
If all the configuration BPDUs have the same root bridge ID, they will be
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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, they will be compared
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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
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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: