User's Manual

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Users Manual of CS-6306R
Chapter 29. UDLD Configuration
Unidirectional Link Detection (UDLD) 29.1
29.1.1 UDLD Overview
UDLD is a L2 protocol that monitors the physical location of the cable through the devices which are
connected by optical cable or twisted-pair, and detects whether the unidirectional link exists. Only when the
connected device supports UDLD can the unidirectional link be detected and shut down. The unidirectional
link can cause a lot of problems, including the STP topology ring. Hence, when detecting a unidirectional link,
UDLD will shut down the affected interface and notify uses.
UDLD works with the physical-layer protocol mechanism to judge the status if the physical link. On the
physical layer, the physical signals and incorrect detections are automatically negotiated and processed,
while UDLD processes other matters, such as detecting the ID of a neighbor and shutting down the incorrect
connection port. If you enable automatic negotiation and UDLD, the detection at layer 1 and layer 2 can
prevent physical/logical links and other protocols’ problems.
29.1.1.1 UDLD Mode
UDLD supports two modes, the normal mode (default) and the aggressive mode. In normal mode, UDLD can
detect the existence of a unidirectional link according to the unidirectional services of the link. In aggressive
mode, UDLD can detect not only the existence of a unidirectional link as in the previous mode but also
connection interruption which cannot be detected by L1 detection protocols.
In normal mode, if UDLD determines that the connection is gone, UDLD will set the state of the port to
undetermined, not to down. In aggressive mode, if UDLD determines that the link is gone and the link
cannot be reconnected, it is thought that interrupted communication is a severe network problem and UDLD
will set the state of the protocol to linkdown and the port is in errdisable state. No matter in what mode, if
UDLD maintains it is a bidirectional link, the port will be set to bidirectional.
In aggressive mode, UDLD can detect the following cases of the unidirectional link:
 On the optical fiber or the twisted pair, an interface cannot receive or transmit services.
 On the optical fiber or the twisted pair, the interface of one terminal is down and the interface of the other
terminal is up.
 One line in the optical cable is broken, and therefore the data can only be transmitted or only be received.
In previous cases, UDLD will shut down the affected interface.