DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
334
Understanding and Troubleshooting the Spanning Tree Protocol
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-3326S 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.
