Today, President Donald Trump signed an executive order ordering the "immediate construction of a physical wall on the southern border" with Mexico.

It was a promise he made repeatedly on the campaign trail, but homeland security expert and host of "The SCIF" podcast Juliette Kayyem said the order does little to lay out a concrete plan about where the wall will go or how it will be funded.

"It's a big nothing," she told Boston Public Radio Wednesday.

"It will not have details on where the wall will be built, because there's nowhere to build it, and it won't have any money tied to it, and it certainly won't require Mexico to pay," Kayyem said.

There is already over 650 miles of border wall in place from previous administrations. Kayyem said geographical barriers and objection from neighboring American states have made constructing additional miles difficult.

"Where there's no wall is because you have a mountain or river, [or] it's tribal lands, which raises other issues, or because the state—in most cases, a red state like Arizona or Texas—doesn't want it there, for a variety of reasons including commercial agricultural reasons," she said.

Funding the wall is another point of contention. While Trump promised on the campaign trail that Mexico would pay for it, he now says U.S. taxpayer dollars will be used for construction, to be reimbursed by Mexico at a later date. Mexico has rejected that idea.

Kayyem was skeptical that Trump would be able to secure Mexican reimbursement for U.S. taxpayer funds. She pointed to Trump's meeting with Mexican President Enrique Peña Nieto in September of last year, after which the then-candidate Trump said he and Nieto had discussed the wall but not who would pay for it.

"He talks tough, and then he gets into a room with the Mexican president and doesn't mention the wall," Kayyem said.

To hear more from Juliette Kayyem, tune in to Boston Public Radio above.