What is UDLD?

Confused about UDLD? Look no further.

What it Stands For

Uni-Directional Link Detection

Why Do We Need It?

Networking consists of both sending and receiving traffic. That is the norm. Relatively rarely do we have instances where we just want to send packets down a link, but not receive any from that same link.

However (with the use of Fibre connections especially) it is possible to have a link fail in only one direction while the other works correctly.

UDLD helps us to detect such faults when they occur.

How Does it Work?

When activated, UDLD sends out frames known as ID Frames out of a port and waits for the connected device to respond with its own ID Frames.

If there is no response, then UDLD can only assume that there is a unidirectional fault on the connection.

So, what next? What happens once a unidirectional fault is detected?

UDLD can work in two ways:

  • Normal: The port in question remains up, but is listed as ‘unknown’
  • Aggressive: The port in question is placed into an errdisable state

How is it Configured?

Lets take a look at how we can configure this on a Switch:

To enable UDLD in Normal mode:

Switch(config-if)#udld enable

To enable UDLD in Aggressive mode:

Switch(config-if)#udld aggressive

To disable UDLD:

Switch(config-if)#udld disable

If you want to change the message time, add ‘message time X’ to the end of either the normal or aggressive commands, an example is below:

Switch(config-if)#udld enable message time 100


Switch(config-if)#udld aggressive message time 100

Bits and Bobs

  • UDLD works on Fibre connections only
  • By default, ID Frames are sent out every 15 seconds (although this can be 7 seconds on some devices)
  • Can be applied globally on the Switch, but will only apply on Fibre ports. For more granular controlĀ better to be done on individual ports


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