I agree a good 'leave' command is essential.

We got our first cat, a now 17 month old Ragdoll, a couple of months ago. Our oldest dog is three and none of our three dogs have lived with cats. They also always barked at them in the garden and would chase them given half the chance.

To start with we made sure the cat could easily get away from them if she needed to, by having a stairgate between the living room and downstairs hallway. Then we introduced them in the same room with the cat unrestrained and the dogs (one at a time at first!) on a lead. They obeyed the 'leave' command, so we tried them off the lad and if they tried to approach her we simply told them to 'leave', which they did.

The cat still isn't their biggest fan, but she certainly isn't bothered by them. She'll happily march about in front of their noses and they know to leave her alone. They pretty much ignore her now. I can't ever see her snuggling up with them though! Oddly enough, they still want to chase other people's cats, so it is not impossible that your dog has live dwith cats before.

I think one of the most important thing is to never allow the opportunity for the cat to be chased by the dog, as the chase itself reinforces the behaviour for the dog. So, if you need to, keep the dog on a lead to start with if the cat is about.