Interlogix NS3550-2T-8S User Manual User Manual
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IFS NS3552-8P-2S AND NS3550-2T-8S User Manual
155
The Hello Time cannot be longer than the Max. Age. Otherwise, a configuration error will
occur.
Max. Age – The Max Age can be from 6 to 40 seconds. At the end of the Max. Age, if a BPDU has still not been received from the
Root Bridge, your Switch will start sending its own BPDU to all other Switches for permission to become the Root Bridge. If it turns
out that your Switch has the lowest Bridge Identifier, it will become the Root Bridge.
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.