H3C Technologies H3C WX5500E Series Access Controllers - Manual (Page 175)

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How STP works
The devices on a network exchange BPDUs to identify the network topology. Configuration BPDUs
contain sufficient information for the network devices to complete spanning tree calculation. A
configuration BPDU includes the following important fields:
• Root bridge ID—Consisting of the priority and MAC address of the root bridge.
• Root path cost—Cost of the 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.
• Designated bridge ID (represented by device priority).
• Designated port ID (represented by port name).
Calculation process of the STP algorithm
1. Initialize the state.
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. Select the optimum configuration BPDU.
Each device sends out its configuration BPDU, and receives configuration BPDUs from other
devices.
Table 56 Selection of the optimum configuration BPDU
Step Actions
1
When the device receives a configuration BPDU on a port, it performs the following
actions:
• 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: