Do you know if I turn on UDLD the 5K and 2K will speak UDLD to each other and do you feel this is a benefit to detect any link issues between the 5K and the FEX?
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The thing is the Nexus 2000s aren't autonomous switches. They are essentially line cards for the 5ks (or 7ks) and have no independent existence, so you can't run the normal protocols across those fex-fabric links.
To my knowledge, you simply can't run UDLD across a fex-fabric port (there's even a bug, CSCub14112, related to the fact that the UDLD config wasn't being removed from fex-fabric ports in certain conditions on the 5ks), though there may be a similar feature embedded in the proprietary control protocol Cisco uses between the 5k and a FEX.
Any Cisco device which can run UDLD can use it with any other Cisco device which can run UDLD. Enabling UDLD can help prevent STP problems. UDLD must be enabled on both devices to actually work, and you should only enable it on ports which are connected to other devices running UDLD.
A lot of Cisco routers don't have UDLD, and some can log "unknown protocol" on some ports where it is connected.
All in all, it is a good practice to run it where you can.