3Com 2928 User Guide - Page 211

Table 1-2 Selection of the optimum configuration BPDU Step Actions Upon receiving a configuration BPDU on a port, the device performs the following: z If the received configuration BPDU has a lower priority than that of the configuration BPDU generated by the port, the device discards the received 1 configuration BPDU and does not process the configuration BPDU of this port. z If the received configuration BPDU has a higher priority than that of the configuration BPDU generated by the port, the device replaces the content of the configuration BPDU generated by the port with the content of the received configuration BPDU. 2 The device compares the configuration BPDUs of all the ports and chooses the optimum configuration BPDU. The following are the principles of configuration BPDU comparison: z The configuration BPDU that has the lowest root bridge ID has the highest priority. z If all the configuration BPDUs have the same root bridge ID, their root path costs are compared. Assume that the root path cost in a configuration BPDU plus the path cost of a receiving port is S. The configuration BPDU with the smallest S value has the highest priority. z If all configuration BPDUs have the same S value, their designated bridge IDs, designated port IDs, and the IDs of the receiving ports are compared in sequence. The configuration BPDU containing a smaller ID wins out. z Selection of the root bridge Initially, each STP-enabled device on the network assumes itself to be the root bridge, with the root bridge ID being its own device ID. By exchanging configuration BPDUs, the devices compare their root bridge IDs to elect the device with the smallest root bridge ID as the root bridge. z Selection of the root port and designated ports on a non-root device The process of selecting the root port and designated ports is as follows: Table 1-3 Selection of the root port and designated ports Step 1 2 Description A non-root device regards the port on which it received the optimum configuration BPDU as the root port. Based on the configuration BPDU and the path cost of the root port, the device calculates a designated port configuration BPDU for each of the rest ports. z The root bridge ID is replaced with that of the configuration BPDU of the root port. z The root path cost is replaced with that of the configuration BPDU of the root port plus the path cost of the root port. z The designated bridge ID is replaced with the ID of this device. z The designated port ID is replaced with the ID of this port. 1-4