D-Link DES-3250TG User Manual page 233

DES-3250TG Layer 2 Fast Ethernet Switch User's Guide
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-3250TG Layer 2 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.
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