D-Link DES-3326 - Manual (Page 275)

DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
state. If the failure exists at boot, STP will not converge and rebooting the bridges will have no effect.
(Note: Rebooting would help temporarily in the previous example).
This type of failure is difficult to detect because the Link-state LEDs for Ethernet links rely on the
transmit side of the cable to detect a link. If a unidirectional failure on a link is suspected, it is usually
required to go to the console or other management software and look at the packets received and
transmitted for the port. A unidirectional port will have many packets transmitted but none received,
or vice versa, for example.
Packet Corruption
Packet corruption can lead to the same type of failure. If a link is experiencing a high rate of physical
errors, a large number of consecutive BPDUs can be dropped and a port in the blocking state would
transition to the forwarding state. The blocking port would have to have the BPDUs dropped for 50
seconds (at the default settings) and a single BPDU would reset the timer. If the MAX AGE is set too
low, this time is reduced.
Resource Errors
The DES-3326 Layer 3 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.
Avoiding Trouble
Know where the root is located.
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