H3C S7500E-X Configuration Manual page 74

Step
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 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, the following attributes
are compared in sequence:
− Designated bridge IDs.
− Designated port IDs.
− IDs of the receiving ports.
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 19 provides an example showing how the STP algorithm works.
Figure 19 The STP algorithm
As shown in
respectively. The path costs of links among the three devices are 5, 10, and 4.
Device state initialization.
1.
In Table
designated bridge ID, and designated port ID.
Table 5 Initial state of each device
Device
Device A
Device B
Actions
configuration BPDU.
Figure 19, the priority values of Device A, Device B, and Device C are 0, 1, and 2,
5, each configuration BPDU contains the following fields: root bridge ID, root path cost,
Port name
Port A1
Port A2
Port B1
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Configuration BPDU on
the port
{0, 0, 0, Port A1}
{0, 0, 0, Port A2}
{1, 0, 1, Port B1}