Cisco Systems N3KC3064TFAL3 Default UDLD Configuration, UDLD Aggressive and Nonaggressive Modes

Configuring Ethernet Interfaces

About the Unidirectional Link Detection Parameter

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	Default Value

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

Globally disabled

Disabled

Enabled on all Ethernet fiber-optic LAN ports

Disabled on all Ethernet 10/100 and 1000BASE-TX LAN ports

UDLD Aggressive and Nonaggressive Modes

UDLD aggressive mode is disabled by default. You can configure UDLD aggressive mode only on point-to-point links between network devices that support UDLD aggressive mode. If UDLD aggressive mode is enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD frames, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.

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 you enable the UDLD aggressive mode, the following occurs:

•One side of a link has a port stuck (both transmission and receive)

•One side of a link remains up while the other side of the link is down

Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)

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