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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter23 Configuring UDLD
Understanding UDLD
In normal mode, UDLD detects 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 port.
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 ports cannot s en d o r re ce ive traffic.
•On fiber-optic or twisted-pair links, one of the ports is down while the othe r is up .
•One of the fiber strands in the cable is disconnected.
In these cases, UDLD disables the affected port.
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 detects whether those fiber strands are connected correctly a nd wheth er tra ffic 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 port 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 a port is disabled and UDLD is running, whenever UDLD is disabled on a port, or
whenever the switch is reset, UDLD clears all existing cache entries for the ports affected by the
configuration change. UDLD sends at least one message to inform t he neighbors to flush the part of
their caches affected by the status change. The message is intend ed to kee p the cache s synchron ized.