I know routers today have the ability to prioritize
traffic, but last I heard, these controls are not
often used for user traffic (let's not discuss
net neutrality here).
Are they used for control (e.g., routing) traffic?
Please say a bit more than "It depends!" 
Our students are interested in real-world practices.
Answers on or off list are welcome.
Christos Papadopoulos
Colorado State University.
Real world answer: It depends.
For instance, Juniper routers auto-police all traffic destined for the main CPU.
Cisco routers (usually) do not. You can configure it, though. Newer ciscos have very nice policing options for traffic to the main CPU. Older ones still have options, but the policing can hurt the router in its own way. There is also some auto-policing in ciscos, e.g. only one ICMP echo request per second per source IP address will be allowed to hit the CPU.
Hope that helps.
Thus spake "Christos Papadopoulos" <christos@CS.ColoState.EDU>
I know routers today have the ability to prioritize traffic, but last I heard,
these controls are not often used for user traffic (let's not discuss
net neutrality here).
They're not often used on _public_ networks for user traffic. They're used extensively on _private_ networks, though, because the people paying the bills for network do so for a particular business purpose and they want to make sure it's met.
Are they used for control (e.g., routing) traffic?
Many routers automatically put control traffic to/from the local node into a separate path that completely bypasses the standard queueing mechanisms (and predates operator-accessible QOS). In other routers, the control plane and forwarding plane are segregated, which achieves the same goal but with a rather different approach.
S
Stephen Sprunk "Those people who think they know everything
CCIE #3723 are a great annoyance to those of us who do."
K5SSS --Isaac Asimov