Dell N1100-ON, Reference Manual

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Layer 2 Switching Commands
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recognize only the sending failures on unidirectional links. If all devices in the
network support UDLD, this functionality is enough to detect all
unidirectional links.
Processing UDLD Traffic from Neighbors
Every UDLD-capable device collects information about all other UDLD-
capable devices. Each device populates UDLD echo packets with collected
neighbor information to help neighbors identify unidirectional links. Every
frame basically contains the device ID of the sender and the collection of
device IDs of its discovered neighbors.
UDLD in Normal-mode
In normal mode, a port’s state is classified as undetermined if an anomaly
exists. These include the absence of its own information in received UDLD
messages or the failure to receive UDLD messages. The state of
undetermined has no effect on the operation of the port. The port is not
disabled and continues operating as it previously did. When in normal mode,
a port is diagnostically disabled for the following cases:
a
UDLD PDU is received from partner that does not have the port’s
own details (echo).
b
When there is a loopback, information sent out on a port is received
back as is.
UDLD in Aggressive-mode
Aggressive mode differs from normal UDLD mode – it can diagnostically
disable a port if the port does not receive any UDLD echo packets after a
bidirectional connection was established. It expands the cases when port can
be disabled. There can be several causes for a port not to receive UDLD
echoes. These include:
• A link is up on one side and down on the other. This can occur on fiber
ports if the transmit port is unplugged on one side.
• Loss of connectivity, i.e. the port is neither transmitting nor receiving, but
the port also reports it is up.