Planet Technology FGSW-2620VM - Manual (Page 45)

FGSW-2620VM User’s Manual
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.
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Notice:
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 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.
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