What is the 'last_name, first_name' format called?

I don't believe there is a well-defined term for this type of list. There first needs to be some clarification of what first name and last name mean - they do not always conform to the presumed model of given name (personal name) and family name (surname).

I have always used the term surname-ordered list in my computer programming. White Pages directories generally describe their own sorting simply as alphabetical, as do most dictionaries.

Sorting by surname has some pitfalls - a number of countries and cultures do fit the standard FIRST_NAME LAST_NAME naming structure. Spanish and Portuguese names often have one or two personal names and one or two family names (such as Vicente Yáñez Pinzón). Chinese and a number of other Asian countries are usually written with the family name first (such as Liu Xiaobo). Some exceptions to the standard sorting methods of names is described nicely in wikipedia. For these reasons, sorting by LAST_NAME can be tricky if LAST_NAME is not a well-defined and independent field.

I believe that is called "Lexicographic Order"