I've never heard of IOS having anything to do with Linux. Cisco owns Linksys now, and most (all?) Linksys routers are based on Linux, but that's no secret.
Cisco branded products run some form of IOS, and IOS has always been Cisco secret sauce. A few years ago, some IOS source code was stolen and parts were made public. I think if any of it contained Linux kernel code the Internet would have erupted into fury.
On a somewhat related point, Juniper JunOS is based on a BSD variant (FreeBSD I think), but there is no way to know how much is BSD and how much was developed by Juniper. It's a consequence of the BSD license.
Cisco branded products run some form of IOS, and IOS has always been Cisco secret sauce. A few years ago, some IOS source code was stolen and parts were made public. I think if any of it contained Linux kernel code the Internet would have erupted into fury.
On a somewhat related point, Juniper JunOS is based on a BSD variant (FreeBSD I think), but there is no way to know how much is BSD and how much was developed by Juniper. It's a consequence of the BSD license.