Operation Manual – MSTPH3C S3610&S5510 Series Ethernet Switches Chapter 1 MSTP Configuration1-2III. Basic concepts in STP1) Root bridgeA tree network must have a root; hence the concept of “root bridge” has beenintroduced in STP.There is one and only one root bridge in the entire network, and the root bridge canchange alone with changes of the network topology. Therefore, the root bridge is notfixed.Upon network convergence, the root bridge generates and sends out configurationBPDUs at a certain interval, and other devices just forward the BPDUs. Thismechanism ensures topological stability.2) Root portOn a non-root bridge device, the root port is the port nearest to the root bridge. The rootport is responsible for communication with the root bridge. A non-root-bridge devicehas one and only one root port. The root bridge has no root port.3) Designated bridge and designated portThe following table describes a designated bridge and a designated port.Table 1-1 Description of designated bridge and designated port:Classification Designated bridge Designated portFor a deviceThe device directlyconnected with this deviceand responsible forforwarding BPDUsThe port through which thedesignated bridgeforwards BPDUs to thisdeviceFor a LANThe device responsible forforwarding BPDUs to thisLAN segmentThe port through which thedesignated bridgeforwards BPDUs to thisLAN 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 Crespectively.z If Device A forwards BPDUs to Device B through AP1, the designated bridge forDevice B is Device A, 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 Bforwards BPDUs to the LAN, the designated bridge for the LAN is Device B, andthe designated port is the port BP2 on Device B.
