20-4
6) Port
ID
A port ID used on a 3Com switch 4500 consists of two bytes, that is, 16 bits, where the first six bits
represent the port priority, and the latter ten bits represent the port number.
The default priority of all Ethernet ports on 3Com switches 4500 is 128. You can use commands to
configure port priorities. For details, see
Configuring Port Priority
.
How STP works
STP identifies the network topology by transmitting configuration BPDUs between network devices.
Configuration BPDUs contain sufficient information for network devices to complete the spanning tree
calculation. Important fields in a configuration BPDU include:
z
Root bridge ID, consisting of root bridge priority and MAC address.
z
Root path cost, the cost of the shortest path to the root bridge.
z
Designated bridge ID, designated bridge priority plus MAC address.
z
Designated port ID, designated port priority plus port number.
z
Message age: lifetime for the configuration BPDUs to be propagated within the network.
z
Max age, lifetime for the configuration BPDUs to be kept in a switch.
z
Hello time, configuration BPDU interval.
z
Forward delay, forward delay of the port.
The implementation of the STP algorithm involves only the following four parts of a configuration BPDU:
z
Root bridge ID
z
Root path cost
z
Designated bridge ID
z
Designated port ID
1) Detailed calculation process of the STP algorithm
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Initial state
Upon initialization of a device, each device 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.
z
Selection of the optimum configuration BPDU
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:
