4-2z Topology change notification (TCN) BPDUs, used for notifying the concerned devices of networktopology changes, if any.Basic Concepts in STPRoot bridgeA tree network must have a root; hence the concept of root bridge was introduced in STP.There is one and only one root bridge in the entire network, and the root bridge can change along withchanges of the network topology. Therefore, the root bridge is not fixed.Upon initialization of a network, each device generates and sends out BPDUs periodically with itself asthe root bridge; after network convergence, only the root bridge generates and sends out configurationBPDUs at a certain interval, and the other devices just forward the BPDUs.Root portOn a non-root bridge, the port nearest to the root bridge is called the root port. The root port isresponsible for communication with the root bridge. Each non-root bridge has one and only one rootport. The root bridge has no root port.Designated bridge and designated portThe following table describes designated bridges and designated ports.Table 4-1 Description of designated bridges and designated ports:Classification Designated bridge Designated portFor a deviceA device directly connected with 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 4-1, AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on Device A, DeviceB, and Device C respectively.z If Device A forwards BPDUs to Device B through AP1, the designated bridge for Device B isDevice A, and the designated port of Device B is port AP1 on Device A.z Two devices are connected to the LAN: Device B and Device C. If Device B forwards BPDUs tothe LAN, the designated bridge for the LAN is Device B, and the designated port for the LAN is theport BP2 on Device B.
