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242 CHAPTER 8: STP OPERATION
Designating Switches
and Ports
A designated switch is a switch in charge of forwarding packets to the local switch
by a port called the designated port. For a LAN, the designated switch is a switch
that forwards packets to the network segment by the designated port.
As illustrated in Figure 1, Switch A forwards data to Switch B through
GigabitEthernet port1/1/1. So to Switch B, the designated switch is Switch A and
the designated port is GigabitEthernet1/1/1 of Switch A. Also, Switch B and
Switch C are connected to the LAN and Switch B forwards packets to the LAN. So
the designated switch of LAN is Switch B and the designated port is
GigabitEthernet1/1/4 of Switch B.
Figure 1 Designated Switch and Designated Port
Calculating the STP
Algorithm
The following example illustrates the calculation process of STP.
The figure1-2 below illustrates the network.
Figure 2 Switch 8800 Networking
To facilitate the descriptions, only the first four parts of the configuration BPDU are
given in the example. They are root ID (expressed as Ethernet switch priority), path
cost to the root, designated switch ID (expressed as Ethernet switch priority) and
the designated port ID (expressed as the port number). As illustrated in the figure
above, the priorities of Switch A, B and C are 0, 1, and 2 and the path costs of
their links are 5, 10, and 4.
Switch A
Switch C
Switch B
LAN
E1/1/1
E1/1/7
E1/1/4
E1/1/2
E1/1/5
E1/1/1
Switch A
with priority 0
S
witch B
w
ith priority 1
Switch C
with priority
2
E1/1/5
E1/1/2
E1/1/1
E1/1/1
E1/1/7
E1/1/4
4
5
10