Cisco Catalyst 2960 Series, Configuration Manual

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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.
Caution
Information About 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.
Normal Mode
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, as long as autonegotiation
is active, the link does not stay up because the Layer 1 mechanisms detects a physical problem with the link.
In this case, UDLD does not take any action and the logical link is considered undetermined.
Related Topics
Enabling UDLD Globally , on page 399
Enabling UDLD on an Interface , on page 400
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
396
Information About UDLD