H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 160)

20-2
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Topology change notification (TCN) BPDUs, used for notifying concerned devices of network
topology changes, if any.
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 20-1 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 20-1 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.
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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.
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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.