Siemens S323, S223 user manual Designated Switch

UMN:CLI	User Manual
	SURPASS hiD 6615 S223/S323 R1.5

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.

		Switch A
		Priority : 8	Root Switch
	Path-cost		Path-cost
Designated	50		100
Switch
	Switch B		Switch C
			Priority : 10
	Priority : 9
	Path-cost		Path-cost
	100		100
	Path 1
	Path 2	Switch D

(PATH 1 = 50 + 100 = 150, PATH 2 = 100 + 100 = 200, PATH 1 < PATH 2, ∴ PATH 1 selected

Fig. 8.11 Designated Switch

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.

202 A50010-Y3-C150-2-7619