H3C Technologies H3C S5560 Series Switches - Manual (Page 95)

61
Basic concepts in STP
Root bridge
A tree network must have a root bridge. The entire network contains only one root bridge, and all the
other bridges in the network are called leaf nodes. The root bridge is not permanent, but can change
with changes of the network topology.
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 the 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
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 20 , 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 and designated port are
as follows:
• The designated bridge for Device B is Device A.
• The designated port of Device B is port A1 on Device A.
If Device B forwards BPDUs to the LAN, the designated bridge and designated port are as follows:
• The designated bridge for the LAN is Device B.
• The designated port for the LAN is port B2 on Device B.