Udld Normal Mode; Udld Aggressive Mode - Cisco ASR 900 Series Configuration Manual

UDLD Overview
The router periodically transmits 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.
UDLD detects and disables unidirectional links on Ethernet fiber and copper interfaces due to miswiring or
malfunctioning of the interfaces.
UDLD is disabled by default on all ports to avoid sending unnecessary traffic.
Note
To configure fibre-optic interfaces, enable the udld command at the global level. For copper interfaces, enable
the udld port command at the interface level.
The figure displays the UDLD mechanism.
Figure 7: Unidirectional Link
UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD can
detect unidirectional links due to misconnected interfaces on fiber-optic connections. In aggressive mode,
UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to
misconnected interfaces on fiber-optic links.

UDLD Normal Mode

In normal mode, UDLD detects the unidirectional link when fiber strands in a fiber-optic interface are
misconnected and the Layer 1 mechanisms do not detect this misconnection. If the interfaces 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 case, the logical link is considered
undetermined, and UDLD does not disable the interface. If one of the fiber strands in a pair is disconnected
and autonegotiation is active, the link does not stay up because the Layer 1 mechanisms did not detect a
physical problem with the link. In this case, UDLD does not take any action, and the logical link is considered
undetermined.

UDLD Aggressive Mode

The UDLD aggressive mode is configured only on the point-to-point link between the network devices that
support the UDLD aggressive mode. With UDLD aggressive mode enabled, a port on a bidirectional link that
has a UDLD neighbor relationship established stops receiving the UDLD packets. The UDLD tries to
re-establish the connection with the neighbor; the port is disabled after eight failed retries.
LAN Switching Configuration Guide Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
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UniDirectional Link Detection (UDLD) Protocol