4-3Figure 4-1 A schematic diagram of designated bridges and designated portsAll the ports on the root bridge are designated ports.Path costPath cost is a reference value used for link selection in STP. By calculating path costs, STP selectsrelatively robust links and blocks redundant links, and finally prunes the network into a loop-free tree.How STP worksThe devices on a network exchange BPDUs to identify the network topology. Configuration BPDUscontain sufficient information for the network devices to complete spanning tree calculation. Importantfields 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 thetransmitting 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 portsto forwarding.
