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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Understanding and Troubleshooting the Spanning Tree Protocol 335
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.