How do I train my American bull dog not to attack my Lab?

Both male. Lab is 7 american bulldog 2. Have been togeather since bulldog was a pup (8wks). They were the best of friends until a few weeks ago when bulldog just snapped and attacked the lab. Now we have to keep them seperated . How can we get the bulldog to stop this behavior. Keeping them seperated is misery for us.