Large dogs don't seem to be much of a problem, they usually don't seem to care about me when I'm cycling. It's the little yippy dogs that like to chase. They're pretty easy to outrun, so I just go ahead and do that. They tend to give up real quick. Large dogs I've always just slowed down to a walking pace and they've always lost interest at that point. The plan has been to dismount and keep the bike between myself and them to fend them off while hopefully waiting for an owner to appear. The plan of last resort is my handgun. Unfortunately due to the laws in the area that I cycle, spray strong enough to affect animals (such as bear spray) and tasers are not legal (considered "weapons" of a type that there is no license available to carry them). If you choose to carry anything, be sure to check your local laws, you may be surprised at what's prohibited!