Web Management Guide

Chapter 12 | Basic Administration Protocols
UDLD Configuration
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Configuring UDLD
Interface Settings
Use the Administration > UDLD (Configure Interface) page to enable UDLD and
aggressive mode which reduces the shut-down delay after loss of bidirectional
connectivity is detected.
Parameters
These parameters are displayed:
◆ Port – Port identifier. (Range: 1-28/54)
◆ UDLD – Enables UDLD on a port. (Default: Disabled)
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UDLD requires that all the devices connected to the same LAN segment be
running the protocol in order for a potential mis-configuration to be
detected and for prompt corrective action to be taken.
■
Whenever a UDLD device learns about a new neighbor or receives a
resynchronization request from an out-of-synch neighbor, it (re)starts the
detection process on its side of the connection and sends N echo messages
in reply. (This mechanism implicitly assumes that N packets are sufficient to
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.
■
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