59• Root bridge ID: consisting of the priority and MAC address of the root bridge.• Root path cost: the cost of the path to the root bridge denoted by the root identifier from thetransmitting bridge.• Designated bridge ID: consisting of the priority and MAC address of the designated bridge.• Designated port ID: consisting of the priority and global port number of the designated port.• Message age: age of the configuration BPDU while it propagates in the network.• Max age: maximum age of the configuration BPDU stored on the switch.• Hello time: configuration BPDU transmission interval.• Forward delay: the delay used by STP bridges to transition port state.Basic concepts in STPRoot bridgeA tree network must have a root bridge.There is only one root bridge in the entire network. The root bridge is not fixed, but can change alongwith changes of the network topology.Upon initialization of a network, each device generates and sends out configuration BPDUs periodicallywith itself as the root bridge. After network convergence, only the root bridge generates and sends outconfiguration BPDUs at a certain interval, and the other devices forward the BPDUs.Root portOn a non-root bridge, the port nearest to the root bridge is the root port. The root port is responsible forcommunication with the root bridge. Each non-root bridge has only one root port. The root bridge has noroot port.Designated bridge and designated portTable 7 Description of designated bridges and designated portsClassification Designated bridge Designated portFor a deviceA device directly connected to the localdevice and responsible for forwardingBPDUs to the local deviceThe port through which thedesignated bridge forwards BPDUsto this deviceFor a LAN The device responsible for forwardingBPDUs to this LAN segmentThe port through which thedesignated bridge forwards BPDUsto this LAN segmentAs shown in Figure 15, both Device B and Device C directly connect to the LAN. If Device A forwardsBPDUs to Device B through port A1, the designated bridge for Device B is Device A, and the designatedport of Device B is port A1 on Device A. If Device B forwards BPDUs to the LAN, the designated bridgefor the LAN is Device B, and the designated port for the LAN is port B2 on Device B.
