D-Link DES-3326 User Manual - Identifying A Data Loop
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Understanding and Troubleshooting the Spanning Tree Protocol
375
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.