The first interview is a primary screening by the hiring manager to just go over the things you say you know and at that point he/she invites you for a panel interview. Second interview was a bit more serious because now you're talking to 5 or more people (7 for mine) and the questions come from every angle. They'll ask scenarios and mostly about the devices you've worked on and how you'd configure the device in a certain scenario. Make no mistake about it, these guys know what they are talking about and if you truly know your stuff, you will get the job.