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BPDU priority in STP mode
The smaller the bridge ID is, the higher the bridge priority is. If the root bridge ID is the same,
then the root path costs are compared. The comparison method is to assume the root path
cost in BPDU and the path cost corresponding to this port to be S, then the BPDU with smaller
S has higher priority.
If the root path costs are the same, compare the designated bridge ID, designated port ID and
ID of the port that receives the BPDU successively, one with the smallest ID has higher priority.
STP computing process
1.
Initial status
Initially, each port of the switch generates a BPDU regarding the switch as the root bridge, with
the root path cost being 0, the ID of the designated bridge being the switch ID, and the
designated port being itself.
2.
Optimal BPDU selection
Each switch sends out its BPDUs and receives BPDUs from other switches. The following table
shows the procedure to select the optimal BPDU.
Step
Content
Receiving BPDU with lower priority: If the priority of the BPDU received by a port is lower than
that of the port itself, the switch discards the received BPDU and does not deal with the BPDU of
that port.
1
Receiving BPDU with higher priority: If the priority of the received BPDU is higher than that of
the port itself, the switch replaces the BPDU of the port with the received one.
2
The switch selects the best BPDU by comparing BPDUs on all ports.
3.
Root bridge selection
The root bridge is selected by BPDU exchange and root bridge ID comparison. The switch with
the smallest root bridge ID is chosen as the root bridge.
4.
Root port and designated port selection
The selection procedure is shown in the following table.
Step
Content
1
For each switch (except the root bridge), the port that receives the optimal BPDU is chosen as
the root port of the switch.
The switch calculates a designated port BPDU for each port according to the root port BPDU and
root port path cost.
2
− The ID of the root bridge is replaced with that of the root port.
Chapter 9 Device management 35