Methods To Detect Unidirectional Links - Cisco Catalyst 3750 Software Configuration Manual

Understanding UDLD
A unidirectional link occurs whenever traffic sent by a local device is received by its neighbor but traffic
from the neighbor is not received by the local device.
In normal mode, UDLD can detect a unidirectional link when fiber strands in a fiber-optic port are
misconnected and the Layer 1 mechanisms do not detect this misconnection. If the ports are connected
correctly but the traffic is one way, UDLD does not detect the unidirectional link because the Layer 1
mechanism, which is supposed to detect this condition, does not do so. In this case, the logical link is
considered undetermined, and UDLD does not disable the interface.
When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected, as long as
autonegotiation is active, the link does not stay up because the Layer 1 mechanisms did not detect a
physical problem with the link. In this case, UDLD does not take any action and the logical link is
considered undetermined.
In aggressive mode, UDLD can detect a unidirectional link by using the previous detection methods.
UDLD in aggressive mode also can detect a unidirectional link on a point-to-point link on which no
failure between the two devices is allowed. It also can detect a unidirectional link when one of these
problems exists:
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In these cases, UDLD shuts down the affected interface.
In a point-to-point link, UDLD hello packets can be considered as a heart beat whose presence
guarantees the health of the link. Conversely, the loss of the heart beat means that the link must be shut
down if it is not possible to re-establish a bidirectional link.
If both fibers are working normally from a Layer 1 perspective, UDLD in aggressive mode detects
whether those fibers are connected correctly and whether traffic is flowing bidirectionally between the
correct neighbors. This check cannot be performed by autonegotiation because autonegotiation operates
at Layer 1.

Methods to Detect Unidirectional Links

UDLD operates by using two mechanisms:
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Catalyst 3750 Metro Switch Software Configuration Guide
20-2
On fiber-optic or twisted-pair links, one of the interfaces cannot send or receive traffic.
On fiber-optic or twisted-pair links, one of the interfaces is down while the other is up.
One of the fiber strands in a pair is disconnected.
Neighbor database maintenance
UDLD learns about other UDLD-capable neighbors by periodically sending a hello packet (also
called an advertisement or probe) on every active interface to keep each device informed about its
neighbors.
When the switch receives a hello message, it caches the information until the age time (hold time or
time-to-live) expires. If the switch receives a new hello message before an older cache entry ages,
the switch replaces the older entry with the new one.
Whenever an interface is disabled and UDLD is running, whenever UDLD is disabled on an
interface, or whenever the switch is reset, UDLD clears all existing cache entries for the interfaces
affected by the configuration change. UDLD sends at least one message to inform the neighbors to
flush the part of their caches affected by the status change. The message is intended to keep the
caches synchronized.
Chapter 20 Configuring UDLD
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