Table 9 Selection of the optimum configuration BPDU
Upon receiving a configuration BPDU on a port, the device performs the following:
If the received configuration BPDU has a lower priority than that of the configuration
BPDU generated by the port, the device discards the received configuration BPDU and
keeps the configuration BPDU this port generated.
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.
The device compares the configuration BPDUs of all the ports and chooses the optimum
The following are the principles of configuration BPDU comparison:
The configuration BPDU with the lowest root bridge ID has the highest priority.
If all configuration BPDUs have the same root bridge ID, their root path costs are compared. For
example, 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.
If all configuration BPDUs have the same root path cost, their designated bridge IDs, designated port
IDs, and the IDs of the receiving ports are compared in sequence. The configuration BPDU that contains
the smallest ID wins.
A tree topology forms upon successful election of the root bridge, the root port on each non-root bridge
and the designated ports.
provides an example of how the STP algorithm works.
Figure 12 Network diagram for the STP algorithm
As shown in
12, the priority of Device A, Device B, and Device C is 0, 1, and 2 respectively, and
the path costs among these links are 5, 10, and 4 respectively.
Initial state of each device