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Configuring UDLD
Prerequisites for UDLD
When configuring the mode (normal or aggressive), make sure that the same mode is configured on both sides of
the link.
Restrictions for UDLD
UDLD is not supported on ATM ports.
A UDLD-capable port cannot detect a unidirectional link if it is connected to a UDLD-incapable port of another
switch.
Loop guard works only on point-to-point links. We recommend that each end of the link has a directly connected
device that is running STP.
Information About UDLD
UDLD
UniDirectional Link Detection (UDLD) is a Layer 2 protocol that enables devices connected through fiber-optic or
twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link
exists. All connected devices must support UDLD for the protocol to successfully identify and disable unidirectional links.
When UDLD detects a unidirectional link, it disables the affected port and alerts you. Unidirectional links can cause a
variety of problems, including spanning-tree topology loops.
Modes of Operation
UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD can detect
unidirectional links due to misconnected ports on fiber-optic connections. In aggressive mode, UDLD can also detect
unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected ports on fiber-optic
links.
In normal and aggressive modes, UDLD works with the Layer 1 mechanisms to learn the physical status of a link. At Layer
1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot
perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both
autonegotiation and UDLD, the Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional
connections and the malfunctioning of other protocols.
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.
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