cisco - vpn one port

Cisco currently sells roughly 30 different models of devices that could be called "routers", and has available several hundred operating system versions for those devices.

"linux router" is so vague as to be nearly meaningless.

If the question is, "Is there at least one Cisco router which, when loaded with an appropriate software version and configured properly, can securely tunnel a single port to a linux-based router that we get to choose that is running software of our choice?", then the answer is YES.

If the question is, "Can all Cisco routers with all software versions securely tunnel ports to all linux-based routers with arbitrary software", then the answer is NO.

If "YES" or "NO" are not the answers you were looking for, then be more specific in your questions.