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In-Depth Information
stop the loop. This can delay the convergence considerably. The latest high-end Catalyst software
implements a feature called BPDU guard that will even disable the port if it is configured for PortFast
and receives a BPDU.
Figure23-12Transient Bridging Loop Because of a Wrong PortFast Configuration
A
A
A
p1
p2
p1
p2
p1
p2
B
B
B
Cable connected
Transient loop:
p1 and p2 forwarding
p2 has received a BPDU
that immediately
blocked it
Awkward STP Parameter Tuning and Diameter Issues
We already saw that an aggressive value for the max age parameter and the forward-delay could lead to
a very unstable STP. The loss of some BPDUs can then cause a loop to appear. Another issue, not very
well known, is related to the diameter of the bridged network. The conservative default values for the
STP impose a maximum network diameter of 7. This means that two distinct bridges in the network
should not be more than seven bridges away the one to the other. Part of this restriction comes from the
Age field BPDUs carry: When a BPDU is propagated from the root bridge toward the leaves of the tree,
the Age field is incremented each time that it goes though a bridge. Eventually, when the Age field of a
BPDU goes beyond the max age, it is discarded. Typically, this will occur if the root is too far away from
some bridges of the network. This issue will affect convergence of the spanning tree.
Software Errors
As mentioned in the introduction to this chapter, the STP is one of the very first features that was
implemented in Cisco products. You can expect this feature to be very stable. Only interaction with new
features, such as EtherChanneling, caused STP to fail in some very specific cases that have been
addressed now. A software bug can be anything, so there is no way of really describing the issue that it
could introduce. Let's simply state again that the most dangerous situation would be to ignore some
BPDUs or, generally speaking, having a blocking port transitioning to forwarding.
