User's Manual

User’s Manual of SGS-5240 Series Managed Switch
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4.2.19.2 Interface Configuration
Switch Management > UDLD > Interface Configuration page is used to enable UDLD and aggressive mode which reduces the
shut-down delay after loss of bidirectional connectivity is detected.
◆ Port – Port identifier. (Range: 1-28/52)
◆ UDLD – Enables UDLD on a port. (Default: Disabled)
 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.
 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).