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'The Ultimate Fighter' vet who went viral for his "let me bang, bro" explosion on 'TUF' won't be bangin' for a bit ... 'cause he's gotta spend 30 days in jail after being arrested for DWI.

Julian Lane -- a cast member on seasons 16 and 25 of the UFC's reality show competition -- was arrested back in October after cops found him asleep at the wheel at an intersection in Mansfield, Ohio.

Cops said Lane was "lethargic and slurring his speech" ... and noted several beer cans and weed in the passenger side of his car.

Julian was transported to the Mansfield police dept., where he blew a .10 ... and charged with 3 misdemeanors -- DWI, driving with a suspended license and possession of marijuana.

The DWI was later reduced to a lesser charge because Lane wasn't physically driving his car when he was rolled on ... and his suspended license charge was dismissed.

But, he'll still have to spend 30 days in the slammer -- starting on April 4.

Lane's scheduled main event fight at Ring of Combat 62 tonight was cancelled ... likely as a result of his arrest.