Illustration Of Stp - Planet FGSW-2620VM User Manual

Port Cost
Default Spanning-Tree Configuration

4.5.3 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.
A value used by STP to evaluate paths –
STP calculates path costs and selects the
path with the minimum cost as the active
path.
Feature
Enable state
Port priority
Port cost
Bridge Priority
The Hello Time cannot be longer than the Max. Age. Otherwise, a configuration error will
occur.
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)
User's Manual of FGSW-Series
200,000-100Mbps Fast Ethernet ports
20,000-1000Mbps Gigabit Ethernet
ports
0 - Auto
Default Value
STP disabled for all ports
128
0
32,768
84