UDLD protocol allows the devices connected through fiber optic or copper (for
example, Category 5 cabling) Ethernet cables that are connected to the LAN
ports to monitor the physical configuration of the cables and detect whether a
unidirectional link exists. When a unidirectional link is detected, the UDLD
shuts down the affected LAN port and alerts the corresponding user, because
unidirectional links cause a variety of problems, including spanning tree
UDLD is a Layer 2
protocol that works with the Layer 1 protocols to determine the physical status
of a link. In Layer 1, auto negotiation takes care of physical signaling and
fault detection. UDLD performs tasks that auto negotiation cannot perform, such
as detecting the identities of neighbors and shutting down misconnected LAN
ports. When you enable both auto negotiation and UDLD, the Layer 1 and Layer 2
detections work together to prevent physical and logical unidirectional
connections and the malfunctioning of other protocols.
A unidirectional link
occurs whenever the traffic transmitted by a local device over a link is
received by a neighbor, but traffic transmitted from the neighbor is not
received by the local device. If one of the fiber strands in a pair is
disconnected, the link does not stay up as long as the auto negotiation is
active. In such a scenario, the logical link is undetermined, and the UDLD does
not take any action. If both the fibers are working normally in Layer 1, the
UDLD in Layer 2 determines whether those fibers are connected correctly and
whether the traffic is flowing bidirectionally between the correct neighbors.
This check cannot be performed by auto negotiation because auto negotiation
operates in Layer 1.
The Cisco ASR 1000
Series Aggregation Services Routers periodically transmit the UDLD packets to
the neighbor devices on LAN ports where UDLD is enabled. If the packets are
echoed back within a specific timeframe and they are lacking a specific
acknowledgment (echo), the link is flagged as unidirectional and the LAN port
is shut down. Devices on both ends of the link must support UDLD for the
protocol to successfully identify and disable the unidirectional links.
By default, the
UDLD is disabled on all ports to avoid sending unnecessary traffic.
The following figure
shows an example of a unidirectional link condition. Switch B successfully
receives traffic from Switch A on the port. However, Switch A does not receive
traffic from Switch B on the same port. UDLD detects the problem and disables
Figure 1. Unidirectional