198 CHAPTER 23: MSTP CONFIGURATION
Basic concepts in STP
1 Root bridge
A tree network must have a root; hence the concept of “root bridge” has been
introduced in STP.
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 at a certain interval, and other devices just forward the
BPDUs. This mechanism ensures topological stability.
2 Root port
On a non-root bridge device, the root port is the port nearest to the root bridge.
The root port is responsible for communication 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
The following table describes a designated bridge and a designated port.
Figure 53 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.
■ 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.
■ 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.
Table 37 Description of designated bridge and designated port
Classification Designated bridge Designated port
For a device The device directly connected with this
device and responsible for forwarding
BPDUs
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
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