(CNN) President Trump wants a wall along the border with Mexico.

Career officials at the agencies most involved in the process, however, are set to recommend a fence—one that will cover only about half the length, CNN has learned.

CNN spoke to more than two dozen sources and experts, including some who are part of high level discussions with the Department of Homeland Security (DHS).

While no plan has been approved or finalized, DHS is currently working on recommendations that Secretary John Kelly will present to Trump.

President Trump has been looking at various blueprints with his advisers, a senior administration official told CNN. He could ultimately insist on a concrete wall stretching across the entire border, as he has promised. It would be a far bigger and a vastly more expensive project, and any plan would need to be sent to Congress for funding approval.

A preliminary internal report by U.S. Customs and Border Protection (CBP), viewed by CNN, outlines various options in phases and uses the words "fence" and "wall/fence."

It also includes a final phase calling for covering more than 1,080 additional miles of border with new barriers which would stretch from coast to coast, with a price tag of $21.6 billion. But sources called that option "only fantasy," and said it is not being seriously considered by officials close to the discussions within the Department of Homeland Security.

Other phases or proposals in the internal report were far more modest, and cheaper.

One senior U.S. Border Patrol official, speaking on the condition of anonymity, told CNN that it's crucial to be able to see through a border barrier. "I'm not calling it a wall because we are talking about a fence that we can look through. That's what we need."

Another senior U.S. Border Patrol official told CNN, "I think fencing has worked for us in the past, but you never want to have a barrier in place that will obstruct your vision, that prevents you from seeing the other side of the border."

Any White House push for a full-length concrete wall would receive little support from career officials within CBP, according to Seth Stodder, a former DHS official who focused on border security under the Obama and Bush administrations. "These are law enforcement agencies. When the White House says 'Jump,' they will say 'How high?' But there will be whispers from them to Congress saying not to do this," Stodder told CNN.

Some sources told CNN one of the biggest challenges will be convincing the president to modify his campaign promise of a solid wall structure. "It's going to be made of hardened concrete, and it's going to be made out of rebar, and steel," Trump said at a December 2015 rally. He repeated the assertion most recently at a news conference in January of this year when he shot back at a reporter who asked him about the fence: "On the fence—it's not a fence. It's a wall. You just misreported it. We're going to build a wall."

Sources tell CNN Trump's vision of a concrete wall is unrealistic, because of cost, timing, and safety.

As one official put it: After the experts convince Trump the wall will not work and it will be a fence, "the White House will just have to figure out a way to spin it [to the public.]"

A concrete wall also poses environmental problems, such as flood risks and hazards for endangered species.

Leading plan

One plan being seriously considered by U.S. Customs and Border Protection, according to sources, calls for upgrading significant portions of the 654 miles of fencing that is currently along the border, as well as the construction of new fencing along key areas.

That option would include about 150 miles of new fence in areas still to be determined, along with about 15 new miles of fencing around San Diego and El Paso, and another 12 new miles around the Rio Grande Valley, which includes Brownsville, Texas. The plan also calls for 272 miles of replacement fencing.

The total area of border covered by fencing under this plan would be about 831 miles, which includes fencing already there now. Under that plan the total barrier between the United States and Mexico would then cover about 831 total miles of a nearly 2000 mile border, still not even half, according to sources.

The new and replaced fencing would cost about $5 billion.

The new fencing would resemble some of the strong fortifications built around the Brownsville, Nogales and San Diego areas. They're called "bollard-style," or "PV1," or "Normandy-style," and essentially involve thick reinforced bars or slats of steel that stand about 18 feet tall, often 6-by-6 inches wide and filled with rebar and concrete for strength, and typically set 6 feet below the ground to prevent tunneling under them.

While the sources did not all agree on the precise length or location of new fencing, all told CNN that protecting the border will require more hi-tech surveillance and more arrests. And all but one of the more than two dozen sources CNN talked to agreed that a concrete wall is not a good idea.

Many officials stressed the need for more border agents, with one saying, "Technology and fencing can only play a part. Personnel have to be in place to respond."

At a House Committee on Homeland Security hearing last week, Secretary Kelly joined several other witnesses who all said that fencing often provides more security to border agents and appeared to back off from using the word "wall."

When asked by Rep. Filemon Vela, a Democrat from Texas, whether "President Trump's promise to build a 2000-mile big beautiful wall" was "a viable option," Secretary Kelly replied, "The President, Congressman, has asked me to take a look at what we need at the Southwest Border and come up with recommendation to him. Yes, there are many places that we need some types of physical barrier right now backed up by men and women at the border protection."

The White House also seems to have given itself some wiggle room in its official definition of the wall. President Trump's executive order on the border wall reads, "It is the policy of the executive branch to secure the southern border of the United States through the immediate construction of a physical wall on the southern border, monitored and supported by adequate personnel so as to prevent illegal immigration, drug and human trafficking, and acts of terrorism."

The order goes on to specify later that "'Wall' shall mean a contiguous, physical wall or other simeilarly secure, contiguous, and impassable physical barrier."

Miami developer Jorge Perez, who has worked with Trump for years, received an email from the Trump Organization with a note from the president himself asking him to get involved with the border project. CNN viewed the email on the condition that it not be shown. It contained sketches of two walls, a primary wall and a secondary wall, with a security road in between. Perez, a Democrat, said he's not going to be involved in the border project.