Illustration Of Stp - GE -DSH-73 User Manual

Chapter 5: Web-Based Management
NOTE: 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.
NOTE: 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.

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
94 GE-DSH-73/DSH-82 and DSH-82-PoE User Manual