3Com 2928 User Guide - Page 212

Step 3 Description The device compares the calculated configuration BPDU with the configuration BPDU on the port of which the port role is to be defined, and acts depending on the comparison result: z If the calculated configuration BPDU is superior, the device considers this port as the designated port, and replaces the configuration BPDU on the port with the calculated configuration BPDU, which will be sent out periodically. z If the configuration BPDU on the port is superior, the device blocks this port without updating its configuration BPDU. The blocked port can receive BPDUs but cannot send BPDUs or forward data. When the network topology is stable, only the root port and designated ports forward traffic, while other ports are all in the blocked state - they receive BPDUs but do not forward BPDUs or user traffic. A tree-shape topology forms upon successful election of the root bridge, the root port on each non-root bridge and the designated ports. The following is an example of how the STP algorithm works. As shown in Figure 1-2, assume that the priority of Device A is 0, the priority of Device B is 1, the priority of Device C is 2, and the path costs of these links are 5, 10 and 4 respectively. Figure 1-2 Network diagram for the STP algorithm z Initial state of each device The following table shows the initial state of each device. Table 1-4 Initial state of each device Device Device A Device B Port name AP1 AP2 BP1 BP2 1-5 BPDU of port {0, 0, 0, AP1} {0, 0, 0, AP2} {1, 0, 1, BP1} {1, 0, 1, BP2}