Which two statements correctly describe UDLD?

Which two statements correctly describe UDLD? (Choose two.)

A.
Unidirectional link detection is achieved through monitoring STP BPDUs.

B.
It is recommended that it be enabled globally rather than on an individual port.

C.
It is recommended that it be used with the loop guard feature.

D.
It is recommended that it be enabled in normal mode.

E.
When an error is detected, normal mode will disable a port.

Explanation:

UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. You can enable the UDLD either globally or on particular interface only. But the best way to enable the UDLD is in global configuration mode:
Example:
R1(Config)#udld enable

UDLD recommended to use with the loop guard feature. The purpose of loop guard is to prevent the alternate and root ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.

Loop guard configuration example:
R1(Config)# spanning-tree loopguard default