3Com MSR 50 Series Configuration Manual page 445

Figure 127 A schematic diagram of designated bridges and designated ports
BP1
Device B
BP 2
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All the ports on the root bridge are designated ports.
4 Path cost
Path cost is a reference value used for link selection in STP. By calculating the path
cost, STP selects relatively "robust" links and blocks redundant links, and finally
prunes the network into a loop-free tree structure.
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 computing. Important fields in a
configuration BPDU include:
Root bridge ID: consisting of root bridge priority and MAC address.
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Root path cost: the cost of the shortest path to the root bridge.
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Designated bridge ID: designated bridge priority plus MAC address.
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Designated port ID, designated port priority plus port name.
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Message age: age of the configuration BPDU while it propagates in a network.
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Max age: maximum age of the configuration BPDU maintained in a device.
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Hello time: configuration BPDU interval.
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Forward delay: forward delay of the port.
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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)
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Root path cost
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Designated bridge ID (in the form of device priority)
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Designated port ID (in the form of port name)
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1 Specific computing process of the STP algorithm
Initial state
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Device A
AP1 AP2
CP1
CP2
LAN
MSTP Overview
Device C
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