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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter23 Configuring UDLD
Understanding UDLD
A unidirectional link occurs whenever traffic sent by a local device is recei v ed by its neighbor b ut traf f ic
from the neighbor is not received by the local device.
In normal mode, UDLD detects a unidirectional link when fiber strands in a fiber-optic interface are
misconnected and the Layer 1 mechanisms do not detect this misconnection. If the interfaces 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 case, the logical link
is considered undetermined, and UDLD does not disable the in te rface .
When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected and auton egotiation
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 detects a unidirectional link by using the previous detection methods. UDLD
in aggressive mode can also detect a unidirectional link on a point-to-point link on which no failure
between the two devices is allowed. It can also detect a unidirectional link when one of these problems
exists:
•On fiber-optic or twisted-pair links, one of the interfaces cannot se nd or r ece ive 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 the cable is disconnected.
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 fiber strands in a cable are working normally from a Layer1 perspective, UDLD in aggressive
mode determines whether those fiber strands are connected correctly and whether traffic is flowing
bidirectionally between the correct neighbors. This check can not be per for med by auto negot iati on
because autonegotiation operates at Layer 1.
Methods to Detect Unidirectional Links
UDLD operates by using two mechanisms:
•Neighbor database maintenance
UDLD learns about other UDLD-capable neighbors by periodi cal ly sen ding a hel lo p acket ( 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 t o inform the neighbors to
flush the part of their caches affected by the status change. The message is intended to keep the
caches synchronized.