H3C LS-3100-52P-OVS-H3 Operation Manual page 238

There is one and only one root bridge in the entire network, and the root bridge can change along with
changes of the network topology. Therefore, the root bridge is not fixed.
After network convergence, the root bridge generates and sends out configuration BPDUs at a certain
interval, and other devices just forward the BPDUs. This mechanism ensures stable topologies.
2) Root port
On a non-root bridge, the port nearest to the root bridge is called the root port. The root port is
responsible for communication with the root bridge. Each non-root bridge has one and only one root
port. The root bridge has no root port.
3) Designated bridge and designated port
The following table describes designated bridges and designated ports.
Table 1-1 Description of designated bridges and designated ports:
Classification
For a device
For a LAN
As shown in Figure 1-1, AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on Device A, Device
B, and Device C respectively.
If Device A forwards BPDUs to Device B through AP1, the designated bridge for Device B is Device
A, and the designated port of Device B is port AP1 on Device A.
Two devices are connected to the LAN: Device B and Device C. 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 the port
BP2 on Device B.
Figure 1-1 A schematic diagram of designated bridges and designated ports
Designated bridge
A device directly connected with the
local device and responsible for
forwarding BPDUs to the local device
The device responsible for forwarding
BPDUs to this LAN segment
The port through which the
designated bridge forwards BPDUs
to this device
The port through which the
designated bridge forwards BPDUs
to this LAN segment
1-2
Designated port