I'm designing a QoS implementation and I'm going to use class-maps and policy-maps and service-policy commands. Now, obviously you use these commands to classify different kinds of traffic and define how you want it handled. But I'm confused about the degree to which certain DSCP markings, by themselves or by default, do or do not result in a certain kind of treatment by routers.
For example, I was reading Cisco's document on DSCP values and they said this about the Assured Forwarding class:
"Assured Forwarding PHB guarantees a certain amount of bandwidth to an AF class and allows access to extra bandwidth, if available."
So if I use clas-maps to define a certain kind of traffic as AF41, but don't configure any other treatment with a policy-map, will the router give that traffic some kind of preferential treatment just because I've marked it as AF41?
I'd have the same question about the EF class.
But at the same time, some classes, such as the CS classes (CS1, CS2, etc.) are comlpetely arbitrary - just labels, essentially - and you define whatever treatment you want, correct?
I guess I'm having trouble differentiating between QoS treatment that I would explicitely configure and QoS treatment that routers will automatically perform, by default, based on certain DSCP values.