The Nexus-focused Packet Pushers were discussing a great question during Cisco Nexus Deep Dive part 2 podcast: do we need LACP on top of UDLD?

Short answer: absolutely.

LACP and UDLD serve two different functions:

• UDLD detects physical link errors and byzantine failures (example: unidirectional fiber link);
• LACP manages the link aggregation groups (LAG, aka port channel) and detects LAG configuration and wiring errors.

Example: LACP can detect a miswired port channel connected to multiple physical switches (that’s why we need MLAG). UDLD can’t do that.

LACP is thus mandatory in a robust network using link aggregation (and after years of yammering, it finally works in vSphere 5.1).

Unfortunately, you can’t tune the LACP timers or timeout values. The 802.1AX standard defines two timer values: short timer (1 second) or long timer (default, 30 seconds) with corresponding timeouts being 3 seconds or 90 seconds.

You can tune UDLD timeouts, with the valid values (in Nexus OS) being between 7 and 90 seconds.

Summary:

• If you need very fast failure detection, use LACP short timers.
• If you need to detect failures within 10-20 seconds, use UDLD.
• Use UDLD (if needed) in combination with LACP on port channels.
• Never ever run port channels without LACP (unless you’re forced to interact with a lobotomized device).