Rande Gerber reveals the secret to his 17-year marriage to Cindy Crawford

Casamigos Tequila founder Rande Gerber and supermodel Cindy Crawford have been married since 1998 -- an impressive 17 years. In the wake of Hollywood's latest big split (unfortunately we're talking about Ben Affleck and Jennifer Garner, of course), a celebrity marriage with that kind of lasting power becomes all the more impressive.

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But Gerber says the secret is one word: friendship.

"We started as friends and we remained friends," he tells ET. "You have to just keep that friendship and fun in the relationship."

Gerber says he and Cindy -- who have two children, Presley and Kaia -- make sure to find time for one another. That includes family vacations to their house in Mexico -- a property they share with Gerber's longtime pal, George Clooney.

Gerber says both he and Clooney got lucky with their ladies, Cindy and Amal. "We're very fortunate," he says.

WATCH: George Clooney Reveals Why He Fell in Love With Amal