Illustration Of Stp - Interlogix GE-DS Series User Manual

GE-DS-82 and NS2503-8P/2C Series User Manual

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.
Figure 4-5-2: Before Applying the STA Rules
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.
In this example, only the default STP values are used.
The switch with the lowest Bridge ID (switch C) was elected the root bridge, and the ports were selected to give a high port cost
between switches B and C. The two (optional) Gigabit ports (default port cost = 20,000) on switch A are connected to one
(optional) Gigabit port on both switch B and C. The redundant link between switch B and C is deliberately chosen as a 100
Mbps Fast Ethernet link (default port cost = 200,000). Gigabit ports could be used, but the port cost should be increased from
the default to ensure that the link between switch B and switch C is the blocked link.
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