Resource Errors; Identifying A Data Loop - D-Link DES-3350SR User Manual

D-Link DES-3350SR Standalone Layer 3 Switch

Resource Errors

The DES-3350SR Layer 2 switch performs its switching and routing functions primarily in hardware, using specialized ASICs.
STP is implemented in software and is thus reliant upon the speed of the CPU and other factors to converge. If the CPU is over-
utilized, it is possible that BPDUs may not be sent in a timely fashion. STP is generally not very CPU intensive and is given
priority over other processes, so this type of error is rare.
It can be seen that very low values for the MAX AGE and the FORWARD DELAY can result in an unstable spanning tree. The
loss of BPDUs can lead to data loops. The diameter of the network can also cause problems. The default values for STP give a
maximum network diameter of about seven. This means that two switches in the network cannot be more than seven hops apart.
Part of this diameter restriction is the BPDU age field. As BPDUs are propagated from the root bridge to the leaves of the spanning
tree, each bridge increments the age field. When this field is beyond the maximum age, the packet is discarded. For large diameter
networks, STP convergence can be very slow.

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.
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