IA and S+ are mandatory for any kind of DOD systems work even most help desk jobs it's mandatory. It's not so much the cert but it's dod ia 8570 (think that's it) at least level 2. CISSP is a funny thing. Soldiers can take it in the military then take the course for free essentially.
You are right though I'd say 80% of the dod is Cisco products these days. They have played a gigantic role in the modernization of it all. So any Cisco cert will help but in most cases not mandatory. Just be ready to know everything about securing a network and be ready to have people be extremely strict about network policies. Some of this stuff people have gone to jail over.
