HP 3100 Series Configuration Manual page 60

The configuration BPDU with the lowest root bridge ID has the highest priority.
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.
If all configuration BPDUs have the same ports value, their designated bridge IDs,
designated port IDs, and the IDs of the receiving ports are compared in sequence. The
configuration BPDU that contains the smallest ID wins.
A tree-shape topology forms when the root bridge, root ports, and designated ports are selected.
Figure 14 describes with an example how the STP algorithm works. This example shows a simplified
spanning tree calculation process.
Figure 14 The STP algorithm
Device A
Priority = 0
Port A1
Port B1
Port B2
Path cost = 4
Device B
Priority = 1
As shown in Figure 14, 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.
Initial state of each device
4.
Table 10 Initial state of each device
Device
Device A
Device B
Device C
NOTE:
In Table 10, each configuration BPDU contains the following fields: root bridge ID, root path cost,
designated bridge ID, and designated port ID.
Comparison process and result on each device
5.
Port A2
Port C1
Port C2
Device C
Priority = 2
Port name
Port A1
Port A2
Port B1
Port B2
Port C1
Port C2
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Configuration BPDU on the
port
{0, 0, 0, Port A1}
{0, 0, 0, Port A2}
{1, 0, 1, Port B1}
{1, 0, 1, Port B2}
{2, 0, 2, Port C1}
{2, 0, 2, Port C2}