How Stp Works - HPE FlexNetwork MSR Series Configuration Manual

Designated bridge and designated port
Classification
For a device
For a LAN
As shown in
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.
Figure 312 Designated bridges and designated ports
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 shortest path to the root bridge.
•
Designated bridge ID—Consisting of the priority and MAC address of the designated bridge.
•
Designated port ID—Designated port priority plus port name.
•
Message age—Age of the configuration BPDU while it propagates in the network.
•
Max age—Maximum age of the configuration BPDU can be maintained on a device.
•
Hello time—Configuration BPDU interval.
Designated bridge
Device directly connected to the local
device and responsible for forwarding
BPDUs to the local device.
Device responsible for forwarding
BPDUs to this LAN segment.
Figure 312, AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on Device A,
Designated port
Port through which the designated bridge
forwards BPDUs to the local device.
Port through which the designated bridge
forwards BPDUs to this LAN segment.
287