Interlogix IFS NS3502-8P-2S - Page 147

IFS NS3502-8P-2S User Manual
147
Forward Delay Timer – The Forward Delay can be from 4 to 30 seconds. This is the time any port on the
Switch spends in the listening state while moving from the blocking state to the forwarding state.
Observe the following formulas when setting the above parameters:
Max. Age _ 2 x (Forward Delay - 1 second)
Max. Age _ 2 x (Hello Time + 1 second)
Port Priority – A Port Priority can be from 0 to 240. 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 200000000. 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 the below diagram. 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.