168 CHAPTER 10: RSTP CONFIGURATION
What are the Designated Bridge and Designated Port?
Figure 50 Designated Bridge and Designated Port
For a Switch, the designated bridge is a Switch in charge of forwarding BPDU to
the local Switch via a port called the designated port. For a LAN, the designated
bridge is a Switch that is in charge of forwarding BPDU to the network segment
via a port called the designated port. As illustrated in
Figure 50, Switch A forwards
data to Switch B via the port AP1. So to Switch B, the designated bridge is Switch
A and the designated port is AP1. Also in the figure above, Switch B and Switch C
are connected to the LAN and Switch B forwards BPDU to LAN. So the designated
bridge of LAN is Switch B and the designated port is BP2.
AP1, AP2, BP1, BP2, CP1 and CP2 respectively delegate the ports of Switch A,
Switch B and Switch C.
The Specific Calculation Process of STP Algorithm
The following example illustrates the calculation process of STP.
Figure 51 illustrates the network.
Figure 51 Switch Networking.
To facilitate the descriptions, only the first four parts of the configuration BPDU
are described in the example. They are root ID (expressed as Ethernet Switch
priority), path cost to the root, designated bridge ID (expressed as Ethernet Switch
priority) and the designated port ID (expressed as the port number). As illustrated
Switch A
Switch C
Switch B
CP2
BP2
CP1
BP1
AP2AP1
LAN
Switch A
with priority 0
Switch C
with priority 2
Switch B
with priority 1
CP2
BP2
CP1
BP1
AP2
AP1
4
10
5
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