Operation Manual – MSTP
H3C S5500-EI Series Ethernet Switches
Chapter 1 MSTP Configuration
1-2
III. 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.
Table 1-1
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
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.
