PLANET FGSW-2840 - Manual (Page 116)

User’s Manual of FGSW-2840 / FGSW-4840S
Default Spanning-Tree Configuration
Feature Default Value
Enable state STP disabled for all ports
Port priority 128
Port cost 0
Bridge Priority 32,768
User-Changeable STA Parameters
The Managed Switch’s factory default setting should cover the majority of installations. However, it is advisable to keep the
default settings as set at the factory; unless, it is absolutely necessary. The user changeable parameters in the Managed Switch
are as follows:
Priority – A Priority for the Managed Switch can be set from 0 to 65535. 0 is equal to the highest Priority.
Hello Time – The Hello Time can be from 1 to 10 seconds. This is the interval between two transmissions of BPDU packets sent
by the Root Bridge to tell all other Switches that it is indeed the Root Bridge. If you set a Hello Time for Managed Switch, and it is
not the Root Bridge, the set Hello Time will be used if and when Managed Switch becomes the Root Bridge.
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, Managed Switch will start sending its own BPDU to all other Switches for permission to become the Root
Bridge. If it turns out that Managed 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
Managed 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.
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