Pedestrians make their way into the the United States from Mexico at the pedestrian border in Nogales, Arizona. Reuters

Donald Trump has promised to deport 11 million undocumented migrants and build a wall along the U.S.-Mexico border. He has said Mexico will pay for the barrier that could harm relations with one of the nation's biggest trade partners.

"I would build a great wall, and nobody builds walls better than me, believe me, and I’ll build them very inexpensively," Trump said, announcing his candidacy on June 16, 2015. "I will build a great, great wall on our southern border. And I will have Mexico pay for that wall."

There are nearly 2,000 miles separating Mexico and the United and about 650 miles already have vehicle and pedestrian fencing, according to a 2016 report from the U.S. Government Accountability Office. Building his wall could require about 339 million cubic feet of concrete, or three times what was used to build the Hoover Dam.

Trump has said his wall could cost up to $12 billion, but a Washington Post study estimated the wall would cost $25 billion. It will be made of precast concrete and reach up to 50 feet. He has said he will force Mexico to pay for the wall by blocking Mexicans who want to send money from the United States back home to Mexico from doing so if they don't show documentation proving their legal status. Mexico receives about $24 billion each year from people sending money from the United States.

"It's an easy decision for Mexico: make a one-time payment of $5 (billion)-10 billion to ensure that $24 billion continues to flow into their country year after year," Trump said on his campaign website.

Trump visited Mexican President Enrique Peña Nieto in Mexico City in August, but said he didn't bring up the wall debate. Before the meeting, Peña Nieto had compared Trump to Adolf Hitler.

"Who pays for the wall? We didn't discuss," Trump said during a news conference following their meeting in Mexico City. "We did discuss the wall. We didn't discuss payment of the wall. That'll be for a later date."

Peña Nieto later said the political leaders had discussed the wall and he had "made it clear" that Mexico wouldn't pay for it. "At the start of the conversation with Donald Trump, I made it clear that Mexico will not pay for the wall," Peña Nieto tweeted. He also called Trump "a threat to Mexico."

There could be other consequences if the wall goes up. Trade between the U.S. and Mexico reached an estimated $583.6 billion in 2015, according to the Office of the U.S. Trade Representative. But that could plummet if a border wall turns away Mexican shoppers who buy food, clothing and other goods in U.S. border towns.

Trump announced his campaign in 2015 by calling many Mexican immigrants "rapists" and "criminals."