D-Link DES-3326 User Manual - Packet Corruption
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
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Understanding and Troubleshooting the Spanning Tree Protocol
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.