1-2
There is one and only one root bridge in the entire network, and the root bridge can change alone with
changes of the network topology. Therefore, the root bridge is not fixed.
Upon network convergence, the root bridge generates and sends out configuration BPDUs periodically.
Other devices just forward the configuration BPDUs received. This mechanism ensures the topological
stability.
2) Root port
On a non-root bridge device, the root port is the port with the lowest path cost to the root bridge. The
root port is used for communicating with the root bridge. A non-root-bridge device has one and only one
root port. The root bridge has no root port.
3) Designated bridge and designated port
Refer to the following table for the description of designated bridge and designated port.
Table 1-1 Designated bridge and designated port
Classification Designated bridge Designated port
For a device
A designated bridge is a device that is
directly connected to a switch and is
responsible for forwarding BPDUs to this
switch.
The port through which the
designated bridge forwards
BPDUs to this device
For a LAN
A designated bridge is a device
responsible for forwarding BPDUs to this
LAN segment.
The port through which the
designated bridge forwards
BPDUs to this LAN segment
Figure 1-1 shows designated bridges and designated ports. In the figure, AP1 and AP2, BP1 and BP2,
and CP1 and CP2 are ports on Device A, Device B, and Device C respectively.
z If Device A forwards BPDUs to Device B through AP1, the designated bridge for Device B is Device
A, and the designated port is the port AP1 on Device A.
z 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 is the port BP2 on
Device B.
Figure 1-1 A schematic diagram of designated bridges and designated ports
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