Very few dogs will defend its pack.
Dogs who attack Snakes are doing so from their prey insticts, prey drive. That's the desire to chase moving objects.
Ray Coppinger has done some interesting research into this. There are basically 7 phases of a hunt:
1. Orient
2. Eye
3. Stalk
4. Chase
5. Grab
6. Bite-Kill
7. Bite-Dissect
Certain breeds of dogs will exhibit different parts of that sequence. As far as I know there is not a single breed in existance that reliably conducts all 7 of those. My first Husky, imo, did. However my current one does not. My GSD will "Eye", "Chase" and "Grab". I've never seen him "Bite-Kill"
I remember seeing a good flow chart on some common breeds and which sections Ray saw them exhibiting. Someone who actually understands canine behaviour will be able to identify which parts their dogs exhibit. Others will think they know, but will be wrong.
I highly suggest that if anyone is interested in this they research Rays work. Or you can remain misinformed.
This might be a good start:
http://www.canis.cl/general/archivos/media/files/Canine Ergonomics - The Science of Working Dogs.pdf
But it's downloading at 500 bytes per second so I can't read it first
This has an alright explanation under "Dog Breeds":
Howstuffworks "How Dogs Work"