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Example of stp calculation – H3C Technologies H3C S12500-X Series Switches User Manual

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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 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 16

provides an example showing how the STP algorithm works.

Figure 16 The STP algorithm

As shown in

Figure 16

, 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 6

, each configuration BPDU contains the following fields: root bridge ID, root path cost,

designated bridge ID, and designated port ID.

Table 6 Initial state of each device

Device

Port name

Configuration BPDU on the

port

Device A

Port A1

{0, 0, 0, Port A1}

Port A2

{0, 0, 0, Port A2}

Device B

Port B1

{1, 0, 1, Port B1}

Device A

Priority = 0

Device B

Priority = 1

Device C

Priority = 2

Port A1

Port A2

Port B1

Port B2

Port C1

Port C2

Path cost = 4

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