1-2STP identifies the network topology by transmitting BPDUs between STP compliant network devices,typically switches and routers. BPDUs contain sufficient information for the network devices to completethe spanning tree calculation.In STP, BPDUs come in two types:z Configuration BPDUs, used to calculate spanning trees and maintain the spanning tree topology.z Topology change notification (TCN) BPDUs, used to notify concerned devices of network topologychanges, if any.Basic concepts in STP1) Root bridgeA tree network must have a root; hence the concept of root bridge has been introduced in STP.There is one and only one root bridge in an entire STP-based network at a given time. But the rootbridge can change because of with changes of the network topology. Therefore, the root bridge is notfixed.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.2) Root portOn a non-root bridge device, the root port is the port with the lowest path cost to the root bridge. Theroot port is used for communicating with the root bridge. A non-root-bridge device has one and only oneroot port. The root bridge has no root port.3) Designated bridge and designated portRefer to the following table for the description of designated bridge and designated port.Table 1-1 Designated bridge and designated portClassification Designated bridge Designated portFor a deviceA designated bridge is a device that isdirectly connected to a switch and isresponsible for forwarding BPDUs to thisswitch.The port through which thedesignated bridge forwardsBPDUs to this deviceFor a LANA designated bridge is a deviceresponsible for forwarding BPDUs to thisLAN segment.The port through which thedesignated bridge forwardsBPDUs to this LAN segmentFigure 1-1 shows designated bridges and designated ports. In the figure, AP1 and AP2, BP1 and BP2,and CP1 and CP2 are ports on Device A, Device B, and Device C respectively.z If Device A forwards BPDUs to Device B through AP1, the designated bridge for Device B is DeviceA, and the designated port is the port AP1 on Device A.z Two devices are connected to the LAN: Device B and Device C. If Device B forwards BPDUs to theLAN, the designated bridge for the LAN is Device B, and the designated port is the port BP2 onDevice B.
