24-port NWay Ethernet Switch User’s Guide
Understanding and Troubleshooting the Spanning Tree Protocol
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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.
