HP FlexNetwork NJ5000, User Manual

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Table 60 Selecting the optimum configuration BPDU
Step Actions
1
Upon receiving a configuration BPDU on a port, the device compares the priority of
the received configuration BPDU with that of the configuration BPDU generated by
the port. It takes one of the following actions:
•
If the former priority is lower, the device discards the received configuration
BPDU and keeps the configuration BPDU the port generated.
•
If the former priority is higher, 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:
a.
The configuration BPDU with the lowest root bridge ID has the highest priority.
b.
If all the 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.
c.
If all configuration BPDUs have the same root bridge ID and S value, their designated
bridge IDs, designated port IDs, and the IDs of the receiving ports are compared in
sequence. The configuration BPDU that contains a smaller designated bridge ID,
designated port ID, or receiving port ID is selected.
A tree-shape topology forms when the root bridge, root ports, and designated ports are selected.
Example of STP calculation
Figure 158 provides an example showing how the STP algorithm works.
Figure 158 STP network
As shown in Figure 158 , the priority values of Device A, Device B, and Device C are 0, 1, and 2, and
the path costs of links among the three devices are 5, 10, and 4, respectively.
1.
Device state initialization.
In
Table 61 , each configuration BPDU contains the following fields: root bridge ID, root path
cost, designated bridge ID, and designated port ID.