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Identifying a Data Loop
Broadcast storms have a very similar effect on the network
to data loops, but broadcast storm controls in modern
switches have (along with subnetting and other network
practices) have been very effective in controlling broadcast
storms. The best way to determine if a data loop exists is to
capture traffic on a saturated link and check if similar
packets are seen multiple times.
Generally, if all the users of a given domain are having
trouble connecting to the network at the same time, a data
loop can be suspected. The port utilization data in the
switch's console will give unusually high values in this case.
The priority for most cases is to restore connectivity as soon
as possible. The simplest remedy is to manually disable all
of the ports that provide redundant links. Disabling ports one
at a time, and then checking for a restoration of the user's
connectivity will identify the link that is causing the problem,
if time allows. Connectivity will be restored immediately after
disabling a data loop.
Avoiding Trouble
Know where the root is located.
Although the STP can elect a root bridge, a well-designed
network will have an identifiable root for each VLAN. Careful
setup of the STP parameters will lead to the selection of this
best switch as the root for each VLAN. Redundant links can
then be built into the network. STP is well suited to
maintaining connectivity in the event of a device failure or
removal, but is poorly suited to designing networks.
Know which links are redundant.
Organize the redundant links and tune the port cost
parameter of STP to force those ports to be in the blocking
state.
For each VLAN, know which ports should be blocking in a
stable network. A network diagram that shows each
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VH-2402-L3 Management Guide 183
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