Interlogix NS3552-8P-2S-V2, User Manual

Page: 139 / 511

Chapter 4: Web configuration
NS3552-8P-2S-V2 User Manual 137
If switch A broadcasts a packet to switch B, switch B broadcasts to switch C, and switch
C broadcasts 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 drops the packet at port
2 and the broadcast ends 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 straightforward.
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.