Fig. 8.11 Designated Switch - Siemens S223 User Manual

UMN:CLI
202
After configuring STP, these switches exchange their information. The priority of SWITCH
A is 8, the priority of SWITCH B is 9 and the priority of SWITCH C is 10. In this case,
SWITCH A is automatically configured as a root switch.
Designated Switch
After deciding a root switch, while SWITCH A transmits packets to SWITCH C, SWITCH A
compares exchanged BPDU to decide the path. The most important information to decide
path is the path-cost. Path-cost depends on transmission rate of LAN interface and path
with lower path-cost is selected.
The standard to decide designated switch is total root path-cost which is added with path-
cost to root. Path-cost depends on transmit rate of switch LAN interface and switch with
lower path-cost is selected to be designated switch.
Path-cost
Designated
Switch
Switch B
Priority : 9
Path-cost
Path 1
Path 2
(PATH 1 = 50 + 100 = 150, PATH 2 = 100 + 100 = 200, PATH 1 < PATH 2, ∴ PATH 1 selected
Designated Switch
Fig. 8.11
In case of the above picture showing SWITCH C sends packet, path-cost of PATH 1 is
150 and path- cost of PATH 2 is total 200(100 + 100 ; path-cost of SWITCH C to B + path-
cost of SWITCH B to C). Therefore lower path-cost, PATH 1 is chosen. In this case, port
connected to Root switch is named Root port. In the above picture, port of SWITCH C
connected to SWITCH A as Root switch is Root port. There can be only one Root port on
equipment.
The standard to decide designated switch is total root path-cost which is added with path-
cost to root. Switch with lower path-cost is selected to be designated switch. When root
path-costs are same, bridge ID is compared.
Switch A
Priority : 8
50
100
Switch D
SURPASS hiD 6615 S223/S323 R1.5
Root Switch
Path-cost
100
Switch C
Priority : 10
Path-cost
100
A50010-Y3-C150-2-7619
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