D-1
Spanning Tree Operation
D
Spanning Tree Operation
Spanning Tree Protocol (STP)
The Switch 208/224 Management Module uses the IEEE 802.1d Spanning Tree
Protocol (STP) to ensure that only one path at a time is active between any
two nodes on the network. In networks where there is more than one physical
path between any two nodes, STP ensures a single active path between them
by blocking all redundant paths.
STP is necessary in such networks because having more than one path
between a pair of nodes causes loops in the network, which can result in a
switch detecting the same node on more than one port. This results in
duplication of messages, leading to a “broadcast storm” that can bring down
the network.
Note The default STP setting in the switch is “Off”. If you are using the switch to
provide redundant network paths, you should reconfigure the Spanning Tree
Protocol to “On”. Refer to “Spanning Tree Configuration” in chapter 4, “Con-
figuring the Switch From the Console” for procedures on enabling STP.
While allowing only one active path through a network at any time, STP retains
any redundant physical path to serve as a backup (blocked) path in case the
existing active path fails. Thus, if an active path fails, STP automatically
activates (unblocks) an available backup to serve as the new active path for
as long as the original active path is down. In the event of a topology change
such as a switch, bridge, or data link failure, STP develops a new spanning
tree that may result in changing some switch ports from the blocking state to
the forwarding state.
For example, see the illustration on the next page.
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