HP 5820X series Configuration Manual page 67

Table 11 Selection of the optimum configuration BPDU
Step Actions
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
1 does not process the configuration BPDU of this port.
• 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 ports and chooses the optimum
2
configuration BPDU.
The following are the principles of configuration BPDU comparison:
The configuration BPDU that has 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
containing the smallest ID wins out.
A tree 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.
Figure 18 Network diagram for the STP algorithm
As shown in Figure 18, 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.
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