4-3
Figure 4-1 A schematic diagram of designated bridges and designated ports
All the ports on the root bridge are designated ports.
Path cost
Path cost is a reference value used for link selection in STP. By calculating path costs, STP selects
relatively robust links and blocks redundant links, and finally prunes the network into a loop-free tree.
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. Important
fields in a configuration BPDU include:
z Root bridge ID: consisting of the priority and MAC address of the root bridge.
z Root path cost: the cost of the path to the root bridge denoted by the root identifier from the
transmitting bridge.
z Designated bridge ID: consisting of the priority and MAC address of the designated bridge.
z Designated port ID: designated port priority plus port name.
z Message age: age of the configuration BPDU while it propagates in the network.
z Max age: maximum age of the configuration BPDU.
z Hello time: configuration BPDU transmission interval.
z Forward delay: the delay used by STP bridges to transit the state of the root and designated ports
to forwarding.
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