Qtech QSW-2900-24T-AC [142/209] Figure 1 a schematic diagram of designated bridges and designated ports

10-140

The following table describes a designated bridge and a designated port.

Table 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 forwards BPDUs to

this LAN segment

Figure 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.

· 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.

· 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.

Figure 1 A schematic diagram of designated bridges and designated ports

& Note:

All the ports on the root bridge are designated ports.

d) How STP works

STP identifies the network topology by transmitting configuration BPDUs between network devices.

Configuration BPDUs contain sufficient information for network devices to complete the spanning tree computing.

Important fields in a configuration BPDU include:

· Root bridge ID: consisting of root bridge priority and MAC address.

· Root path cost: the cost of the shortest path to the root bridge.

· Designated bridge ID: designated bridge priority plus MAC address.

· Designated port ID, designated port priority plus port name.