WATCH: Ex-border protection chief explains exactly why Trump's wall is 'not going to work'
Gil Kerlikowske, the former head of the Customs and Border Protection agency, talks with ABC News (Screen cap).

President Trump's top campaign pledge was to build a giant wall across the U.S.-Mexico border -- but one former border security official says it's a complete waste of resources.

ABC News recently interviewed Gil Kerlikowske, who retired from his role as commissioner of the Customs and Border Protection agency last week, and asked him what he thought about Trump's plan to build a border wall. In short, he was not at all impressed.

"I think that anyone who’s been familiar with the southwest border and the terrain... kind of recognizes that building a wall along the entire southwest border is probably not going to work," he said.

Among other things, Kerlikowske said that a wall wouldn't stop most of the illegal immigration that Trump claims to be concerned about, as more immigrants come into the U.S. through other points of entry.

Additionally, a sizable chunk of people now in the country illegally are people who initially entered the country legally, but then overstayed their visas.

What's more, Kerlikowske doesn't think Trump or his team understand the difficult logistics and high expenses that will come with constructing and maintaining the wall.

"When we look at the cost -- and we have about 600 miles of fencing now -- we look at the maintenance and the upkeep, we know how incredibly difficult it is," he said.

Check out the whole video below.