I got an interesting question from Andrew:

Would you say that bridge assurance makes UDLD unnecessary? It doesn't seem clear from any resource I've found so far (either on Cisco's docs or on Google)."

It’s important to remember that UDLD works on physical links whereas bridge assurance works on top of STP (which also implies it works above link aggregation/port channel mechanisms). UDLD can detect individual link failure (even when the link is part a LAG); bridge assurance can detect unaggregated link failures, total LAG failure, misconfigured remote port or a malfunctioning switch.

Some of the reasons you might want to prefer UDLD would thus include:

• UDLD can detect link problems even when you disable spanning tree (for example, on layer-2 data center interconnect links);
• If you cannot use BFD, you can run UDLD on routed interfaces to detect link problems faster than the routing protocol hello mechanisms would;
• You can run UDLD on individual ports of a static port channel (for example, redundant ports in a layer-2 DCI).

The benefits of Bridge Assurance are also obvious:

• It can detect STP configuration problems;
• You can use it in a mixed-vendor environment, whereas you can use UDLD only between a pair of Cisco’s switches.

In complex designs, you’ll probably end up using both.

More information

To learn more about BFD, read the article I wrote about it in early 2000s. You’ll find it somewhere in this list.