Methods To Detect Unidirectional Links - Cisco IE-4000 Software Configuration Manual

Configuring UDLD
Information About UDLD
In normal mode, UDLD detects a unidirectional link when fiber strands in a fiber-optic port are misconnected and the
Layer 1 mechanisms do not detect this misconnection. If the ports are connected correctly but the traffic is one way,
UDLD does not detect the unidirectional link because the Layer 1 mechanism, which is supposed to detect this condition,
does not do so. In this case, the logical link is considered undetermined, and UDLD does not disable the port.
When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected, as long as autonegotiation is active,
the link does not stay up because the Layer 1 mechanisms detects a physical problem with the link. In this case, UDLD
does not take any action and the logical link is considered undetermined.
In aggressive mode, UDLD detects a unidirectional link by using the previous detection methods. UDLD in aggressive
mode can also detect a unidirectional link on a point-to-point link on which no failure between the two devices is allowed.
It can also detect a unidirectional link when one of these problems exists:

On fiber-optic or twisted-pair links, one of the ports cannot send or receive traffic.

On fiber-optic or twisted-pair links, one of the ports is down while the other is up.

One of the fiber strands in the cable is disconnected.
In these cases, UDLD disables the affected port.
In a point-to-point link, UDLD hello packets can be considered as a heart beat whose presence guarantees the health
of the link. Conversely, the loss of the heart beat means that the link must be shut down if it is not possible to reestablish
a bidirectional link.
If both fiber strands in a cable are working normally from a Layer 1 perspective, UDLD in aggressive mode detects
whether those fiber strands are connected correctly and whether traffic is flowing bidirectionally between the correct
neighbors. This check cannot be performed by autonegotiation because autonegotiation operates at Layer 1.

Methods to Detect Unidirectional Links

UDLD operates by using two methods:

Neighbor database maintenance
UDLD learns about other UDLD-capable neighbors by periodically sending a hello packet (also called an
advertisement or probe) on every active port to keep each device informed about its neighbors.
When the switch receives a hello message, it caches the information until the age time (hold time or time-to-live)
expires. If the switch receives a new hello message before an older cache entry ages, the switch replaces the older
entry with the new one.
Whenever a port is disabled and UDLD is running, whenever UDLD is disabled on a port, or whenever the switch is
reset, UDLD clears all existing cache entries for the ports affected by the configuration change. UDLD sends at least
one message to inform the neighbors to flush the part of their caches affected by the status change. The message
is intended to keep the caches synchronized.

Event-driven detection and echoing
UDLD relies on echoing as its detection mechanism. Whenever a UDLD device learns about a new neighbor or
receives a resynchronization request from an out-of-sync neighbor, it restarts the detection window on its side of
the connection and sends echo messages in reply. Because this behavior is the same on all UDLD neighbors, the
sender of the echoes expects to receive an echo in reply.
If the detection window ends and no valid reply message is received, the link might shut down, depending on the
UDLD mode. When UDLD is in normal mode, the link might be considered undetermined and might not be shut down.
When UDLD is in aggressive mode, the link is considered unidirectional, and the port is disabled.
If UDLD in normal mode is in the advertisement or in the detection phase and all the neighbor cache entries are aged
out, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbors.
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