Planet WGSW-2840 [38/104] Lan 2 lan 3

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.

Bridge ID = 15

Port 3

Port 1

Port 2

Bridge ID = 20

Port 1

Port 2

Port 3

Bridge ID = 30

Port 1

Port 2

Port 3

LAN 1

LAN 2 LAN 3

Portcast = 19

Portcast = 19 Portcast = 19

Portcast = 4

Portcast = 19

Portcast = 4

Before Applying the STA Rules

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