As i understand there are 5 main concepts in QoS
1.Classification
2.Marking
3.Queuing
4. Shaping
5.Policing
I also understand that Cisco says for every platform (almost) there is default queuing mechanism exist , for example -
For 3850 it is 2P6Q3T queuing model , so if we deploy Cisco validated design for 2P6Q3T the whole QoS gets covered for this platform and we don’t need to do anything.
Now My Question is if we use such queuing model where does the classification and Marking go ? are they part of these queuing model ?
next small question is : I read somewhere that Cat 3850/3650 only support Egress queuing, what does that mean ? why not they support ingress queuing ?
2P6Q3T is a queuing model. Thus, by default it encompasses only the Queuing concept of QoS. It does not include classification, marking shaping and policing. Marking and classification must be implemented in addition to this queuing model.
The 3850/3650 devices only support egress queuing. If however you want to implement some sort of QoS on ingress ports, it is possible to do so using other QoS mechanisms as described in this Cisco Documentation.
The 2P6Q3T uses the DSCP markings on traffic to implement its queuing policies. If nothing else is configured, then the default markings for the traffic are used. If these are tweaked, then of course the changes are taken into account.