UDLD - fiber (Enforce), AOC (Enforce), DAC (Alert only)?

[RWelch]

Trying to verify something I might have goofed up in my past network configurations?  UDLD should be used when connecting switch < - - > switch scenarios when using:

Fiber (SFP or SFP+) = Enforce?

AOC = Enforce?

DAC = Alert only?

Thank you for any feedback you might offer.

[Brash · Accepted solution]

From my understanding you're correct - UDLD is used to prevent loops due to unidirectional links to other switches. It won't really be of benefit having it enabled on client facing ports.

The benefits of it are primarily for optical fiber connections - definitely SFP, maybe AOC but can't hurt.

While it's likely not really needed for DAC, there's no harm in having it set for alert.

[RWelch]

Thank you - initially I connected downstream switches to a distribution switch using fiber and later replaced that with an appropriate length DAC, however I forgot to change it back to Alert only.

[PhilipDAth]

Just a note, in the past I have had UDLD enforcement cause more outages from falsely triggering, than outages saved by it working correctly.

It should be usable on any point-to-point link, or any type.  It is most popular with fibre, though, where a single fibre failure in a pair can result in a unidirectional link.

[PhilipDAth]

ps. I would only use it when you have redundant links between switches.  So you can sustain a single link failure.

If you only have a single link - enforcement is a bit pointless, because if the only link fails, you are going to have an outage either way.