Planet Technology WSD-800 3. Illustration of STP

User’s Manual of WSD-800 / WGSD-910/WGSD-1022C

Max. Age _ 2 x (Hello Time + 1 second)

Port Priority – A Port Priority can be from 0 to 255. The lower the number, the greater the probability the port will be chosen as the Root Port.

Port Cost – A Port Cost can be set from 0 to 65535. The lower the number, the greater the probability the port will be chosen to forward packets.

3. Illustration of STP

A simple illustration of three switches connected in a loop is depicted in Figure 5-7. In this example, you can anticipate some major network problems if the STP assistance is not applied. If switch A broadcasts a packet to switch B, switch B will broadcast it to switch C, and switch C will broadcast it to back to switch A ... and so on. The broadcast packet will be passed indefinitely in a loop, potentially causing a network failure. In this example, STP breaks the loop by blocking the connection between switch B and C. The decision to block a particular connection is based on the STP calculation of the most current Bridge and Port settings. Now, if switch A broadcasts a packet to switch C, then switch C will drop the packet at port 2 and the broadcast will end there. Setting-up STP using values other than the defaults, can be complex. Therefore, you are advised to keep the default factory settings and STP will automatically assign root bridges/ports and block loop connections. Influencing STP to choose a particular switch as the root bridge using the Priority setting, or influencing STP to choose a particular port to block using the Port Priority and Port Cost settings is, however, relatively straight forward.

	LAN 1
	Portcast = 19
A	Port 3
Bridge ID = 15
Port 1	Port 2
Portcast = 4	Portcast = 4

	Portcast = 4		Portcast = 4
B	Port 1		C	Port 1
Bridge ID = 30			Bridge ID = 20
Port 2	Port 3	Portcast = 19	Port 2	Port 3
	Portcast = 19			Portcast = 19
	LAN 2			LAN 3

Before Applying the STA Rules

In this example, only the default STP values are used.

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