UDLD Overview
Configuring UDLD
page 2-4
OmniSwitch AOS Release 7 Network Configuration Guide
June 2013
UDLD Overview
UDLD is a Layer 2 protocol used to examine the physical configuration connected through fiber-optic or
twisted-pair Ethernet cables. When a port is affected and only a unidirectional link is working, UDLD
detects and administratively shuts down the affected port, and alerts the user. Unidirectional links can
create
hazardous situations such as Spanning-Tree topology loops caused, for instance, by unwiring of
fiber strands, interface malfunctions, faults of the media converter, and so on.
The UDLD feature is supported on the following port types:
•
Copper ports
•
Fiber ports
UDLD Operational Mode
UDLD supports two modes of operation:
•
Normal mode
•
Aggressive mode
UDLD works with the Layer 1 mechanisms to determine the physical status of a link. A unidirectional link
occurs whenever the traffic sent from 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 unidirectional.
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.