What Are Possible Causes for Failure in Learning ND Entries?

In V100R002C00 and later versions, the possible causes for failure in learning ND entries are as follows:
The physical status or protocol status of the interface is Down. You can run the display ipv6 interface interface-type interface-number command in any view to check the physical status and protocol status of the interface.
The STP state of the interface is Discarding. You can run the display stp brief command in any view to check the STP state of the interface.
The number of ND entries or IPv6 routes exceeds the limit.
You can run the display ipv6 neighbors command in any view to check information about ND entries.
You can run the display ipv6 routing-table statistics command in any view to check statistics information about IPv6 routes.