3Com 2928 User Guide - Page 209

Root port On a non-root bridge, the port nearest to the root bridge is called the root port. The root port is responsible for communication with the root bridge. Each non-root bridge has one and only one root port. The root bridge has no root port. Designated bridge and designated port The following table describes designated bridges and designated ports. Table 1-1 Description of designated bridges and designated ports: Classification Designated bridge Designated port For a device A device directly connected with the local device and responsible for forwarding BPDUs to the local device The port through which the designated bridge forwards BPDUs to the local device For a LAN The device responsible for forwarding BPDUs to this LAN segment The port through which the designated bridge forwards BPDUs to this LAN segment As shown in Figure 1-1, 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 Device 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 to the LAN, the designated bridge for the LAN is Device B, and the designated port for the LAN is the port BP2 on Device B. Figure 1-1 A schematic diagram of designated bridges and designated ports Path cost Path cost is a reference value used for link selection in STP. By calculating path costs, STP selects relatively robust links and blocks redundant links, and finally prunes the network into a loop-free tree. 1-2