Designated bridge and designated port
1.Description of designated bridges and designated ports
Classification | Designated bridge | Designated port | |
| A device directly connected with the local | The port through which the designated | |
For a device | device and responsible for forwarding | ||
bridge forwards BPDUs to this device | |||
| BPDUs to the local device | ||
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| The device responsible for forwarding | The port through which the designated | |
For a LAN | bridge forwards BPDUs to this LAN | ||
BPDUs to this LAN segment | |||
| segment | ||
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As shown in a, 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 of Device B is 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 for the LAN is the port BP2 on Device B.
a.A schematic diagram of designated bridges and designated ports
Path cost
Path cost is a reference value used for link selection in STP. STP calculates path costs to select the most robust links and block redundant links that are less robust, to prune the network into a
NOTE:
All the ports on the root bridge are designated ports.
How STP works
The devices on a network exchange BPDUs to identify the network topology. Configuration BPDUs contain sufficient information for the network devices to complete spanning tree calculation. Important fields in a configuration BPDU include:
∙Root bridge ID: Comprises the priority and MAC address of the root bridge.
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