I have a border collie cross (cross golden retriever). She's very protective and faithful and yes, she is very 'nippy'. In fact, when we got her from the rescue kennel she came with a warning to that effect. She will not tolerate people she doesn't know trying to pet her - will snap at them, so I have to warn them not to pet her, no matter how friendly she may seem. Basically, people have to wait for her to approach them if they are to get onto 'cuddling' terms with her. Fortunately, most of my friends are sensible dog owners.
She can be like it with other dogs too. In recent months she has begun to play more with the dogs we meet regularly whilst out walking, but even so, play is on the understanding that she sets the rules (she's a bossy little madam). If she doesn't want to play, she will say so in no uncertain terms.
Her background is unknown to us - we got her as a nine-year-old and she'd arrived at the kennels as a stray. Her behaviour on occasions would suggest that she hasn't been very kindly treated in the past (cowers if she's peed on the floor or raided the bin, for instance), so I know this isn't, perhaps, typical behaviour. But I'll hazard a guess that her dubious background is what brings out the worst in her personality (although she's lovely, is my best girlie mate, and I wouldn't part with her for anything).