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Spanning Tree Protocol
Packet Corruption
Packet corruption can also lead to Spanning Tree Protocol 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 are 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 ProLiant BL e-Class C-GbE Interconnect 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 in order 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.
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
hops. 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.
Identifying a Data Loop
Broadcast storms have a very similar effect on the network to data loops, but broadcast storm
controls in modern switches (along with subnetting and other network practices) have been
very effective in controlling broadcast storms. The best way to determine if a data loop exists
is to capture traffic on a saturated link and check if similar packets are seen multiple times.
Generally, if all the users of a given domain are having trouble connecting to the network at
the same time, a data loop can be suspected. The port utilization data in the switch’s console
will give unusually high values in this case.
The priority for most cases is to restore connectivity as soon as possible. The simplest remedy
is to manually disable all of the ports that provide redundant links. Disabling ports one at a
time, and then checking for a restoration of the user’s connectivity will identify the link that
is causing the problem, if time allows. Connectivity will be restored immediately after
disabling a data loop.
D-12 HP ProLiant BL e-Class C-GbE Interconnect Switch User Guide
HP CONFIDENTIAL Codename: DeLorean Part Number: 263682-002 Last Saved On: 2/5/03 10:46 AM