Manual

93
User Guide
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If the Root Bridge ID of X equals that of Y, but the Root Path cost of X is smaller than
that of Y, X is superior to Y and X will become the Root Bridge.
If the Root Bridge ID and the Root Path cost of X equal those of Y, but the Bridge ID of X
is smaller than that of Y, X is superior to Y and X will become the Root Bridge.
If the Root Bridge ID, the Root Path cost and Bridge ID of X equal those of Y, but the
Port ID of X is smaller than that of Y, X is superior to Y and X will become the
Root Bridge.
STP Generation

Starting STP
After initially configuring STP each Switch considers itself the Root Bridge and gener-
ates a configuration BPDU for each Port as a Root Port. The root path cost will be 0, the
ID of the Designated Bridge being that of the Switch itself and the Designated
Port being itself.

Comparing BPDUs
Each Switch sends out configuration BPDUs and receives configuration BPDUs on one
or more of its own Ports from other Switches. The following table shows the
comparison operations.
Step Operation
1
If the priority of the BPDU received on the Port is lower than that of the BPDU of the
Port itself, the Switch discards the BPDU and does not change the BPDU of the Port.
2
If the priority of the BPDU is higher than that of the BPDU of the Port itself, the Switch
replaces the BPDU of the Port with the BPDU received and compares it with those of
other Ports on the Switch to find the Port with the highest priority.
Table 7-1 Comparing BPDUs

Selecting the Root Bridge
The Root Bridge is selected by comparing BPDUs. The Switch with the lowest Root ID is
chosen as the Root Bridge.

Selecting the Root Port and Designated Port
Step Operation
1
For each Switch (except the Switch chosen as the Root Bridge) in a Network, the
Port that receives the BPDU with the highest priority is chosen as the Root Port
of the Switch.