How Stp Works - HP 1910 User Manual

Designated bridge and designated port
Table 54 Description of designated bridges and designated ports:
Classification
For a device
For a LAN
As shown in
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.
Device B and Device C are connected to the LAN. 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.
Figure 176 Designated bridges and designated ports
NOTE:
All the ports on the root bridge are designated ports.
Path cost
Path cost is a reference value used for link selection in STP. By calculating path costs, STP selects relatively
robust links and blocks redundant links, and finally prunes the network into a loop-free tree.

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. A
configuration BPDU includes the following important fields:
Root bridge ID—Consisting of the priority and MAC address of the root bridge.
•
Root path cost—Cost of the path to the root bridge.
•
Designated bridge
A device directly connected to the local
device and responsible for forwarding
BPDUs to the local device.
The device responsible for forwarding
BPDUs to this LAN segment.
Figure 176, AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on Device A, Device
Designated port
The port through which the designated
bridge forwards BPDUs to the local
device.
The port through which the designated
bridge forwards BPDUs to this LAN
segment.
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