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 Operation
1 If the priority of the BPDU received on the port is lower than that of the BPDU if 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 received one and compares it with those of
other ports on the switch to obtain the one with the highest priority.
Table 7-1 Comparing BPDUs
Selecting the root bridge
The root bridge is selected by BPDU comparing. The switch with the smallest root ID is chosen as
the root bridge.
Selecting the root port and designate port
The operation is taken in the following way:
Step Operation
1 For each switch (except the one 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.
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 is
chosen as the designated port and the BPDU of this port is replaced with the
resulting BPDU. The port regularly sends out the resulting BPDU;
If the BPDU of this port takes the precedence over the resulting BPDU, the BPDU
of this port is not replaced and the port is blocked. The port only can receive
BPDUs.
Table 7-2 Selecting root port and designated port
Tips：
In a STP with stable topology, only the root port and designated port can forward data, and the
other ports are blocked. The blocked ports only can receive BPDUs.
RSTP (Rapid Spanning Tree Protocol), evolved from the 802.1D STP standard, enable Ethernet
ports to transit their states rapidly. The premises for the port in the RSTP to transit its state rapidly
are as follows.
The condition for the root port to transit its port state rapidly: The old root port of the switch
stops forwarding data and the designated port of the upstream switch begins to forward
data.
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