7-3
Figure 7-1
A schematic diagram of designated bridges and designated ports
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 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 bridges on a network exchange BPDUs to identify the network topology. Configuration BPDUs
contain sufficient information for the bridges 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.
