H3C Technologies H3C WX5500E Series Access Controllers - Manual (Page 66)

55
Basic concepts in STP
This section describes the basic concepts in STP.
Root bridge
A tree network must have a root bridge.
There is only one root bridge in the entire network. The entire network contains only one root bridge. All
the other bridges in the network are called "leaf nodes". The root bridge is not permanent, but can
change when the network topology changes.
Upon initialization of a network, each device generates and periodically sends configuration BPDUs with
itself as the root bridge. After network convergence, only the root bridge generates and periodically
sends configuration BPDUs. The other devices only forward BPDUs.
Root port
On a non-root bridge, the port nearest to the root bridge is the root port. The root port communicates with
the root bridge. Each non-root bridge has only one root port. The root bridge has no root port.
Designated bridge and designated port
Table 7 Description of designated bridges and designated ports
Classification Designated bridge Designated port
For a device
Device directly connected with the local
device and responsible for forwarding BPDUs
to the local device.
Port through which the designated
bridge forwards BPDUs to this device.
For a LAN
Device responsible for forwarding BPDUs to
this LAN segment.
Port through which the designated
bridge forwards BPDUs to this LAN
segment.
As shown in Figure 14 , Device B and Device C are directly connected to a LAN. If Device A forwards
BPDUs to Device B through Port A1, the designated bridge for Device B is Device A, and the designated
port of Device B is Port A1 on Device A. 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 Port B2 on Device B.
Figure 14 Schematic diagram of designated bridges and designated ports