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9-3
Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide
OL-16506-17
Chapter 9 UniDirectional Link Detection (UDLD) Protocol
How to Configure the UDLD Protocol
Figure 9-1 Unidirectional Link
Configuring the UDLD Aggressive Mode
Configure the UDLD aggressive mode 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.
To prevent spanning tree loops, nonaggressive UDLD with the default interval of 15 seconds is fast
enough to shut down a unidirectional link before a blocking port transitions to the forwarding state (with
default spanning tree parameters).
When the UDLD aggressive mode is enabled, the UDLD will error disable the ports on the link to prevent
the traffic from being discarded under the following scenarios:
• One side of a link has a port (either Tx and Rx) stuck.
• One side of a link remains up while the other side of the link has gone down.
Default UDLD Configuration
Table 9-1 shows the default UDLD configuration.
How to Configure the UDLD Protocol
These sections describe how to configure the UDLD protocol:
• Enabling UDLD Globally, page 9-4
TX
TX
RX
RX
Switch A
Switch B
18720
Table 9-1 UDLD Default Configuration
Feature Default Value
UDLD global enable state Globally disabled
UDLD aggressive mode Disabled
UDLD per-port enable state for fiber-optic media Disabled
UDLD per-port enable state for twisted-pair
(copper) media
Disabled on all Ethernet 10/100 and
1000BASE-TX LAN ports