67
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
provides an example showing how the STP algorithm works.
Figure 18 The STP algorithm
As shown in
, 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
, each configuration BPDU contains the following fields: root bridge ID, root path cost,
designated bridge ID, and designated port ID.
Table 5 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.
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
