H3C Technologies H3C S5560 Series Switches - Manual (Page 98)

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Example of STP calculation
Figure 21 provides an example showing how the STP algorithm works.
Figure 21 The STP algorithm
As shown in Figure 21 , the priority values of Device A, Device B, and Device C are 0, 1, and 2,
respectively. The path costs of links among the three devices are 5, 10, and 4.
1. Device state initialization.
In
Table 7 , each configuration BPDU contains the following fields: root bridge ID, root path cost,
designated bridge ID, and designated port ID.
Table 7 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}
Port B2 {1, 0, 1, Port B2}
Device C
Port C1 {2, 0, 2, Port C1}
Port C2 {2, 0, 2, Port C2}
2. Configuration BPDUs comparison on each device.
In
Table 8 , each configuration BPDU contains the following fields: root bridge ID, root path cost,
designated bridge ID, and designated port ID.
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