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Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide
OL-16506-17
Chapter 9 UniDirectional Link Detection (UDLD) Protocol
Information About the UDLD Protocol
• High Availability (HA) is not supported, but when the Ethernet port is up and UDLD is enabled on
the port, the UDLD automatically performs the detection.
• Only Gigabit Ethernet, 10 Gigabit Ethernet, and Fast Ethernet interfaces are supported.
• Supports only the basic UDLD functions.
Information About the UDLD Protocol
These sections describe how UDLD works:
• UDLD Overview, page 9-2
• Configuring the UDLD Aggressive Mode, page 9-3
• Default UDLD Configuration, page 9-3
UDLD Overview
The Cisco-proprietary 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 topology loops.
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.
Note By default, the UDLD is disabled on all ports to avoid sending unnecessary traffic.
Figure 9-1 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 the port.