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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 VH-2402-L3 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.
182 VH-2402-L3 Management Guide
9033691-01
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