(CNN) President Donald Trump is following through with one of the first pledges he made a year and a half ago when he announced his long-shot bid for the White House -- directing federal resources toward building a wall along the southern border.

But the reality of building the wall could be more difficult than the rallying cries would suggest. The length of the wall, when construction will begin, how much it will cost and who will pay for it remain fundamental questions.

There is also ongoing debate about the feasibility of the structure and the need for it in the first place.

Trump's push for a wall dates back to the day in June 2015 when he announced his campaign and promised to "build a great, great wall on our southern border" and said Mexico would pay.

"Mark my words," he said for added effect.

His effort to turn those words into reality is expected to be formally announced Wednesday and is among several immigration-related actions anticipated in a multi-day rollout from the new administration.

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Since he first made the pledge, walling off our neighbors to the south became an oft-repeated promise and rallying cry, both for the candidate and his supporters. Chants of "build that wall" thundered through arenas and convention halls. In at least one instance, Trump himself led the cheer. Building "an impenetrable physical wall on the southern border, on day one" is listed as item No. 1 on his "10 Point Plan to Put America First."

Reality of building the wall

At the first press briefing of the new administration Monday, one of the first questions to White House Press Secretary Sean Spicer was about the wall. Spicer said the administration was working with agency heads and Congress to move forward on the project. He offered no further details.

Not everyone is as enthusiastic as the President about building a wall, including the man whose job it was to secure the U.S. border for the past three years.

"I don't see any efficacy in building a wall across the border," said Gil Kerlikowske, who until last week served as Commissioner of Customs and Border Protection under former President Barack Obama.

"The border and migration issues are just unbelievably complex," Kerlikowske said in an interview with CNN. "And a simple answer to a complex problem is most assuredly the wrong answer."

Trump has yet to appoint Kerlikowske's replacement.

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Kerlikowske said the rugged terrain in the Arizona desert and the shifting Rio Grande River in Texas, which routinely changes depth and even direction, both represent natural obstacles to building a border wall. Some of the land along the border in the Rio Grande Valley is privately owned, representing another challenge, he said.

Kerlikowske also noted that the vast majority of people trying to enter the US are actually showing up at border entry points and seeking asylum or turning themselves in. Most are not trying to scale the fence in the dark of night, he said. Even if a wall was built, he said, it would require a significant hiring increase to have enough agents to monitor the wall. That would be no easy feat for an agency that is currently 1,200 employees below full strength.

Kerlikowske said that, in addition to some 700 miles of fencing, the border is currently patrolled by agents on foot, bikes, motorcycles, ATVs and horseback. There's also an air wing, unmanned predator drones, ground sensors, infrared video and tower-mounted video with a range of 25 miles.

"It's all preferable to a wall," Kerlikowske said. "Unless you monitor that wall."

On the campaign trail, Trump told a crowd of supporters in Anaheim, California, in May that he'd reached out to leaders in the union representing Border Patrol agents and asked whether a wall was truly needed.

"Mr. Trump, It's absolutely vital," came the reply, he told the crowd. "It's an absolutely important tool. Maybe our most important tool to stop what's going on."

Hearing that, Trump told the crowd:

"We're going to build the wall. We have no choice."

At the point, the crowd began chanting, "build that wall." Trump joined in from the podium.

The National Border Patrol Council, the union representing Border Patrol agents, endorsed Trump for President -- marking the first time the group had taken such an action.

Brandon Judd, president of the council, said in a recent interview with CNN, that Trump was "by far the best on border security" when compared with other candidates.

Judd said he didn't reach that decision based on his pledge to build a wall. He said his group favored "barriers in strategic locations."

But, importantly, he didn't say those barriers had to come in the form of a wall as Trump is suggesting.

'Doesn't have to be a wall'

"It doesn't have to be a wall," Judd said.

He called the double fencing along the border in the San Diego area "insanely effective."

Judd, who said he met with Trump and his transition team, argued such fencing allows agents to confront people on the US side of the border while they still have a fence in front of them to slow their progress.

"He was very much willing to listen to our perspective," Judd said. "It was, you know: 'I didn't realize that'. And 'that's good to know'."

Trump would then turn to an aide, Judd recounted, and say: "Hey, get on this. Get back with me. I need to know more about this."

Judd said he was confident his group's views were being taken into consideration.

"If you look at the areas they're currently surveying -- if you look at what the talk is, a lot of that is coming from us," he said. "They absolutely 100% are in the process of implementing things we suggested."

Judd said he signed a non-disclosure agreement that prohibited him from going into detail about planned construction.

Shawn Moran, vice president of the Border Patrol union, said he thinks most people would be surprised to learn that it's possible to drive for 30 minutes in some Border Patrol sectors and not encounter agents on patrol.

"I think a lot of Americans think you're going to see agents in towers every hundred yards," Moran said. "That doesn't exist."

He said one instance in which a wall may have made a difference was in the 2009 slaying of Border Patrol Agent Robert Rosas. Rosas was shot to death by assailants who crawled under fencing near the eastern San Diego County town of Campo.

The US-Mexico border is approximately 2,000 miles long, running from San Diego to Brownsville, Texas. Trump initially said he wanted to wall off the entire border, but has since said that 1,000 miles may do.

Few details

He has offered few details on the wall since being elected president. But speaking at a news conference earlier this month, he reiterated his pledge, referring to the border as "an open sieve." He made construction sound imminent.

"Mike Pence is leading an effort to get final approvals through various agencies and through Congress for the wall to begin," he told reporters on Jan. 11.

CNN reported earlier this month that Trump transition team officials had been in discussions with the US Army Corps of Engineers and the Interior Department to begin planning a wall, including how certain environmental laws could come into play.

"It seems clear they were trying to size up the environmental laws that may be obstacles to building the wall," said one U.S. official familiar with the inquiry.

A review of Trump's statements on the campaign trail offers a glimpse of what he envisions -- or did at the time.

Trump said his wall would be solid, strong and "probably 35 to 40 feet up in the air."

"It will actually look good," he added. "You know, as good as a wall is going to look."

The dimensions of the border barricade grew when former Mexican President Vicente Fox insisted his country would never pay for it.

"It just got 10 feet taller," Trump told CNN's Wolf Blitzer last year.

When then-Republican presidential contender Jeb Bush referred to the proposed wall as a fence in August of 2015, Trump took to Twitter to correct him:

"It's not a fence, Jeb, it's a WALL," Trump tweeted, "and there's a BIG difference!"

In an interview with MSNBC in February, then-candidate Trump said he'd priced out the project at about $8 billion. Later that month, he told Fox News' Sean Hannity the cost would be $10 billion.

"It's going to be a very terrific wall," Trump said. "$10 billion is not bad."

Hannity asked Trump, whose name is synonymous with his brand as a builder and hotelier whether the name Trump will be on the wall.

"Only if its' beautiful," he replied.