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Basic concepts in STP
Root bridge
A tree network must have a root bridge.
There is only one root bridge in the entire network. The root bridge is not fixed, but can change along
with changes of the network topology.
Upon initialization of a network, each device generates and sends out configuration BPDUs periodically
with itself as the root bridge. After network convergence, only the root bridge generates and sends out
configuration BPDUs at a certain interval, and the other devices forward the BPDUs.
Root port
On a non-root bridge, the port nearest to the root bridge is the root port. The root port is responsible for
communication with the root bridge. Each non-root bridge has only one root port. The root bridge has no
root port.
Designated bridge and designated port
Table 9 Description of designated bridges and designated ports
Classification Designated bridge Designated port
For a device
A device directly connected to the local
device and responsible for forwarding
BPDUs to the local device
The port through which the
designated bridge forwards BPDUs
to this device
For a LAN
The device responsible for forwarding
BPDUs to this LAN segment
The port through which the
designated bridge forwards BPDUs
to this LAN segment
As shown in Figure 17, both Device B and Device C directly connect to the LAN. If Device A forwards
BPDUs to Device B through port A1, the designated bridge for Device B is Device A, and the designated
port of Device B is port A1 on Device A. If Device B forwards BPDUs to the LAN, the designated bridge
for the LAN is Device B, and the designated port for the LAN is port B2 on Device B.
Figure 17 A schematic diagram of designated bridges and designated ports
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