Information Technology Reference
In-Depth Information
Figure23-11Bridging Loop in a Unidirectional Link Scenario
B
blocking
A
B unblocks its port and can
forward traffic this way...
BPDU lost this way
Here, let's suppose that the link between bridges A and B is unidirectional and drops traffic from A to
B while transmitting traffic from B to A. Suppose that Bridge B should be blocking. We already
mentioned that a port can block only if it receives BPDUs from a bridge that has a better priority. In this
case, all these BPDUs coming from Bridge A are lost, and Bridge B eventually forwards traffic, creating
a loop. Note that, in this case, if the failure exists at startup, the STP will not converge correctly. This
means that rebooting the bridges will have absolutely no effect (whereas it could temporarily help in the
previous case).
Cisco introduced the UDLD protocol on high-end switches. This feature is capable of detecting wrong
cabling or unidirectional links on Layer 2 and automatically breaks resulting loops by disabling some
ports. It is really worth running wherever possible in a bridged environment.
Packet Corruption
Packet corruption can also lead to the same kind of failure. If a link is experiencing a high rate of
physical errors, a certain number of consecutive BPDUs could be lost, leading a blocking port to
transition to forwarding. This case is rather seldom because STP default parameters are very
conservative. The blocking port would need to miss its BPDUs for 50 seconds before transitioning to
forwarding, and a single BPDU successfully transmitted would break the loop. This case specially
occurs when STP parameters have been adjusted without care (max age reduced, for instance).
Resource Errors
Even on high-end switches that perform most of their switching functions in hardware using specialized
Asics, STP is implemented in software. This means that if the CPU of the bridge is overutilized for any
reason, it is possible that it lacks resources to send out BPDUs. The STA is generally not very
processor-intensive and has priority over other processes. You will see in the upcoming section “Look
for Resource Errors” that there are some guidelines that govern the number of instances of STP that a
particular platform can handle.
PortFast Configuration Error
PortFast is a feature that you typically will want to enable for a port connected to a host. When the link
comes up on this port, the first stages of the STA are skipped and the port directly transitions to the
forwarding mode. This can obviously be dangerous when not used correctly. Loops occurs then when
moving a cable and
should
be transient only.
In Figure 23-12, Bridge A is a bridge with port p1 already forwarding and port p2 configured for
PortFast. Bridge B is a hub. As soon as the second cable is plugged into Bridge A, p2 goes to forwarding
and creates a loop between p1 and p2. This will stop as soon as p1 or p2 receives a BPDU that will put
one of these two ports in blocking modes. The problem with that kind of transient loop is that if the
looping traffic is very intensive, the bridge may have trouble successfully sending the BPDU that will
