142 CHAPTER 14: MSTP CONFIGURATION
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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 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:
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When the network topology is stable, only the root port and designated ports
forward traffic, while other ports are all in the blocked state - they only receive STP
packets but do not forward user traffic.
Once the root bridge, the root port on each non-root bridge and designated
ports have been successfully elected, the entire tree-shaped topology has been
constructed.
Tabl e 97 Selection of the root port and designated ports
Step Description
1 A non-root-bridge device takes the port on which the optimum configuration BPDU
was received as the root port.
2 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.
3 The device compares the calculated configuration BPDU with the configuration
BPDU on the port whose role is to be determined, and acts as follows based on the
comparison result:
■ If the calculated configuration BPDU is superior, this port will serve as the
designated port, and the configuration BPDU on the port will be replaced with
the calculated configuration BPDU, which will be sent out periodically.
■ If the configuration BPDU on the port is superior, the device stops updating the
configuration BPDUs of the port and blocks the port, so that the port only
receives configuration BPDUs, but does not forward data or send configuration
BPDUs.
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