3Com 4210 PWR Configuration Manual page 143

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:
Root bridge ID, consisting of root bridge priority and MAC address.
■
Root path cost, the cost of the shortest path to the root bridge.
■
Designated bridge ID, designated bridge priority plus MAC address.
■
Designated port ID, designated port priority plus port name.
■
Message age: lifetime for the configuration BPDUs to be propagated within the
■
network.
Max age, lifetime for the configuration BPDUs to be kept in a switch.
■
Hello time, configuration BPDU interval.
■
Forward delay, forward delay of the port.
■
n
For the convenience of description, the description and examples below involve
only four parts of a configuration BPDU:
Root bridge ID (in the form of device priority)
■
Root path cost
■
Designated bridge ID (in the form of device priority)
■
Designated port ID (in the form of port name)
■
1 Detailed calculation process of the STP algorithm
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.
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:
Table 96 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
■
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
■
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.
STP Overview 141