Default Udld Configuration - Cisco nexus 5000 series Configuration Manual

Configuring Ethernet Interfaces
autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected
LAN ports. When you enable both autonegotiation and UDLD, 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 traffic transmitted by the local device over a link is received by the
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, and if autonegotiation is active, the link does not stay up. In this case, the logical
link is undetermined, and UDLD does not take any action. If both fibers are working normally at Layer 1,
then UDLD at Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing
bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation, because
autonegotiation operates at Layer 1.
A Cisco Nexus device periodically transmits UDLD frames to neighbor devices on LAN ports with UDLD
enabled. If the frames are echoed back within a specific time frame and they lack 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 in order for the protocol to successfully identify and disable unidirectional links.
The following figure shows an example of a unidirectional link condition. Device B successfully receives
traffic from Device A on the port. However, Device A does not receive traffic from Device B on the same
port. UDLD detects the problem and disables the port.
Figure 1: Unidirectional Link

Default UDLD Configuration

The following table shows the default UDLD configuration.
Table 2: UDLD Default Configuration
Feature
UDLD global enable state
UDLD aggressive mode
UDLD per-port enable state for fiber-optic media
UDLD per-port enable state for twisted-pair (copper)
media
OL-25842-01
Cisco Nexus 5000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.1(3)N1(1)
Unidirectional Link Detection Parameter
Default Value
Globally disabled
Disabled
Enabled on all Ethernet fiber-optic LAN ports
Enabled
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