H3C Technologies H3C MSR 50 - Manual (Page 333)

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• Root path cost—Cost of the shortest path to the root bridge.
• Designated bridge ID—Consisting of the priority and MAC address of the designated bridge.
• Designated port ID—Designated port priority plus port name.
• Message age—Age of the configuration BPDU while it propagates in the network.
• Max age—Maximum age of the configuration BPDU can be maintained on a device.
• Hello time—Configuration BPDU interval.
• Forward delay—Delay used by STP bridges to transit the state of the root and designated ports to
forwarding.
For simplicity, the descriptions and examples in this document involve only the following fields in the
configuration BPDUs:
• Root bridge ID (represented by device priority)
• Root path cost (related to the rate of the link connecting the port)
• Designated bridge ID (represented by device priority)
• Designated port ID (represented by port name)
Calculation process of the STP algorithm
1. State initialization.
When you initialize 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.
2. Optimum configuration BPDU selection.
Each device sends out its configuration BPDU and receives configuration BPDUs from other
devices.
Table 139 Optimum configuration BPDU selection
Step Actions
1
Upon receiving a configuration BPDU on a port, the device performs the following:
• If the received configuration BPDU has a lower priority than that of the configuration
BPDU generated by the port, the device discards the received configuration BPDU and
does not process 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 replaces 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.
Configuration BPDU comparison uses the following principles:
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, their root path costs are compared.
For example, the root path cost in a configuration BPDU plus the path cost of a receiving port is
S. The configuration BPDU with the smallest S value has the highest priority.
If all configuration BPDUs have the same S value, their designated bridge IDs, designated port
IDs, and the IDs of the receiving ports are compared in sequence. The configuration BPDU
containing a smaller ID is selected.
3. Root bridge selection.