H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 52)

43
• Configuration BPDUs, used for calculating a spanning tree and maintaining the spanning tree
topology.
• Topology change notification (TCN) BPDUs, which notify network devices of network topology
changes.
A configuration BPDU contains the following information for network devices to complete spanning tree
calculation.
• Root bridge ID: Comprises the priority and MAC address of the root bridge.
• Root path cost: The cost of the path to the root bridge.
• Designated bridge ID: Comprises the priority and MAC address of the designated bridge.
• Designated port ID: Comprises the port priority and global port number.
• Message age: The times that the configuration BPDU has been forwarded on the network.
• Max age: The maximum age of the configuration BPDU.
• Hello time: The transmission interval of the configuration BPDU.
• Forward delay: The delay before a port transitions to the forwarding state.
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 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 out configuration BPDUs
with itself as the root bridge. After network convergence, only the root bridge generates and periodically
sends out 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
Table 7 Description of designated bridges and designated ports
Classification Designated bridge Designated port
For a device
A device directly connected with the local
device and responsible for forwarding BPDUs
to the local device
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
As shown in Figure 10 , 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.