UDLD Overview Configuring UDLD
page 38-4 OmniSwitch 6800/6850/9000 Network Configuration Guide March 2008
UDLD Overview
UDLD is a Layer 2 protocol used to examine the physical configuration connected through fiber-optic or
twisted-pair Ethernet cables. UDLD detects and administratively shuts down the affected port, and alerts
the user when a unidirectional link exists. Unidirectional links can create hazardous situations such as
Spanning-Tree topology loops caused, for instance, by unwiring of fiber strands, interface malfunctions,
media converter’s faults, etc.
The UDLD feature is supported on the following port types:
• Copper ports
• Fiber ports
UDLD Operational Mode
UDLD supports two modes of operation: normal and aggressive modes. UDLD works with the Layer 1
mechanisms to determine the physical status of a link. A unidirectional link occurs whenever the traffic
sent by a local device is received by its neighbor; but the traffic from the neighbor is not received by the
local device.
Normal Mode
In this mode, the protocol depends on explicit information instead of implicit information. If the protocol
is unable to retrieve any explicit information, the port is not put in the shutdown state; instead, it is marked
as Undetermined. The port is put in the shutdown state only when it is explicitly determined that the link is
defective when it is determined on the basis of UDLD-PDU processing that link has become unidirec-
tional. In any such state transition, a trap is raised.
Aggressive Mode
In this mode, UDLD checks whether the connections are correct and the traffic is flowing bidirectionally
between the respective neighbors. The loss of communication with the neighbor is considered an event to
put the port in shutdown state. Thus, if the UDLD PDUs are not received before the expiry of a timer, the
port is put in the UDLD-shutdown state. Since the lack of information is not always due to a defective
link, this mode is optional and is recommended only for point-to-point links.
UDLD shuts down the affected interface when one of these problems occurs:
• 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 the cable is disconnected.
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