46Configuration BPDUs contain sufficient information for the network devices to complete spanning treecalculation. Important fields in a configuration BPDU include the following:• Root bridge ID—Comprises the priority and MAC address of the root bridge.• Root path cost—The cost of the path to the root bridge.• Designated bridge ID—Comprises the priority and MAC address of the designated bridge.• Designated port ID—Comprises the port priority and global port number.• Message age—The times that the configuration BPDU has been forwarded on the network.• Max age—The maximum age of the configuration BPDU.• Hello time—The transmission interval of the configuration BPDU.• Forward delay—The delay before a port transitions to the forwarding state.Basic concepts in STPRoot bridgeA tree network must have a root bridge.There is only one root bridge in the entire network. The entire network contains only one root bridge. Allthe other bridges in the network are called "leaf nodes". The root bridge is not permanent, but canchange with changes of the network topology.Upon initialization of a network, each device generates and periodically sends configuration BPDUs withitself as the root bridge. After network convergence, only the root bridge generates and periodicallysends configuration BPDUs. The other devices only forward the BPDUs.Root portOn a non-root bridge, the port nearest to the root bridge is the root port. The root port communicates withthe root bridge. Each non-root bridge has only one root port. The root bridge has no root port.Designated bridge and designated portTable 7 Description of designated bridges and designated portsClassification Designated bridge Designated portFor a deviceA device directly connected with the localdevice and responsible for forwarding BPDUsto the local deviceThe port through which the designatedbridge forwards BPDUs to this deviceFor a LAN The device responsible for forwarding BPDUsto this LAN segmentThe port through which the designatedbridge forwards BPDUs to this LANsegmentAs shown in Figure 12, Device B and Device C are directly connected to a 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.
