Web Management Guide-R01

Chapter 13 | Basic Administration Protocols
UDLD Configuration
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get through a link and reach the other end, even though some of them
might get dropped during the transmission.)
Since this behavior must be the same on all the neighbors, the sender of
the echoes expects to receive an echo in reply. If the detection process
ends without the proper echo information being received, the link is
considered to be unidirectional.
◆ Aggressive Mode – Reduces the shut-down delay after loss of bidirectional
connectivity is detected. (Default: Disabled)
UDLD can function in two modes: normal mode and aggressive mode.
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In normal mode, determination of link status at the end of the detection
process is always based on information received in UDLD messages:
whether that’s information about the exchange of proper neighbor
identification or the absence of such. Hence, albeit bound by a timer,
normal mode determinations are always based on gleaned information,
and as such are “event-based.” If no such information can be obtained (e.g.,
because of a bidirectional loss of connectivity), UDLD follows a
conservative approach to minimize false positives during the detection
process and deems a port to be in “undetermined” state. In other words,
normal mode will shut down a port only if it can explicitly determine that
the associated link is faulty for an extended period of time.
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In aggressive mode, UDLD will also shut down a port if it loses bidirectional
connectivity with the neighbor for the same extended period of time (as
that mentioned above for normal mode) and subsequently fails repeated
last-resort attempts to re-establish communication with the other end of
the link. This mode of operation assumes that loss of communication with
the neighbor is a meaningful network event in itself, and a symptom of a
serious connectivity problem. Because this type of detection can be event-
less, and lack of information cannot always be associated to an actual
malfunction of the link, this mode is recommended only in certain
scenarios (typically only on point-to-point links where no communication
failure between two neighbors is admissible).
◆ Operation State – Shows the UDLD operational state (Disabled, Link down,
Link up, Advertisement, Detection, Disabled port, Advertisement - Single
neighbor, Advertisement - Multiple neighbors)
◆ Port State – Shows the UDLD port state (Unknown, Bidirectional,
Unidirectional, Transmit-to-receive loop, Mismatch with neighbor state
reported, Neighbor's echo is empty)
The state is Unknown if the link is down or not connected to a UDLD-capable
device. The state is Bidirectional if the link has a normal two-way connection to
a UDLD-capable device. All other states indicate mis-wiring.
◆ Message Interval – The interval between UDLD probe messages used for the
indicated operational state.