Device Management
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STP Computing Process
1. Initial Status
Initially, each switch regards itself as the root, and generates a configuration BPDU for each
port on it as a root, with the root path cost being 0, the ID of the designated bridge being that
of the switch, and the designated port being itself.
2. BPDU Comparison
Each switch sends out configuration BPDUs and receives a configuration BPDU on one of
its ports from another switch. The following table shows the comparing operations.
Step Content
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.
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 received one.
2
The switch selects the best BPDU by comparing BPDUs on all ports.
3. Select the root bridge
The root bridge is selected by BPDU comparing. The switch with the smallest root ID will be
chosen as the root bridge.
4. Select the root port and designated port
The operation is taken in the following way:
Step Content
1
Non-Root Bridge will receive the port of the best BPDU as the root port.
2
Using the root port BPDU and the root path cost, the switch generates a
designated port BPDU for each of its ports.
Root ID is replaced with that of the root port;
Root path is replaced with the sum of the root path cost of the root port and the
path cost between this port and the root port;
The ID of the designated bridge is replaced with that of the switch;
The ID of the designated port is replaced with that of the port.
3
The switch compares the resulting BPDU with the BPDU of the desired port whose
role you want to determine.
If the resulting BPDU takes the precedence over the BPDU of the port, the port
