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In my setup above (Cisco 2960 running IOS15), I have all devices marked in red in vlan 20 (WLAN_VLAN), devices in green in VLAN 10 (PC_VLAN) and devices in yellow which are VoIP in the voice vlan. I am attempting to create a vlan based QoS such that if an access point and a PC all connected to the same switch forward frames or packets and they reach that switch, the packet from the PC gets higher priority than the access point, so it travels out of the switch first to its destination. This is my attempt.....

!in global configuration mode

    mls qos
!
!
!on the switch port where the access point is connected

    interface FastEthernet0/3
     switchport mode access
     switchport access vlan 10
     mls qos cos 4
     mls qos trust cos
!
!
!
!on the switch port where the PC is connected

    interface FastEthernet0/4
     switchport access vlan 20
     switchport mode access
     mls qos cos 3
     mls qos trust cos
!

I simulated this in packet tracer but it does not seem to work. How do I go about it? what am I doing wrong?

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    You need to understand a couple of things. First, Packet Tracer is not what you use to simulate ral-world things. It is not real IOS, it is only a simple application to get you through the CCNA, and it is missing many features of real equipment. Second, QoS only works in congestion, and switch queues are tiny and do not normally starve other queues, even if some have priority. QoS in not a substitute for the required bandwidth.
    – Ron Maupin
    Aug 1 at 15:16
  • @RonMaupin Fine. I want to know whether my configuration to achieve QoS for the hosts are correct?
    – user78234
    Aug 1 at 16:50
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As stated by Ron Maupin, packet tracer does not have full support for functions.

To answer your question directly- your configuration is incomplete. QoS works in two parts. The first part is marking- which you have completed- however not efficiently as you are marking everything at COS 4 and COS 3. Some traffic classes are data hungry and may result in queue starvation for other data classes. Things such as file transfers could eat up an entire queue where an HTTP request- while bursty- is relatively short lived. While you only have 8 classes to work with when dealing with CoS you have to understand that QoS goes a lot deeper and there are many layers that can happen.

(further expansion on part 1): you could set the COS as an initial identifier and then at your L3 (router or switch) set your class-maps/policies to further analyze the traffic and break into further groups so that file transfers are put into AF22 for COS4, control traffic (DNS, DHCP, etc) is set to AF33, etc. RFC4594 provides standard marking recommendations to follow.

The second portion of QoS is policing/shaping. Without a police/shape function going on- you are simply applying a marking. Think of it like this- marking tells QoS WHAT the traffic is- and the police/shape function tells how to manage it. Without marking- everything is best effort (no better or worse than what it is without QoS) You also have to remember that Cisco only defines ONE single priority queue. All classes that are set as the priority queue share the same logical queue. Refer to RFC4594 for priority queue best practice.

To enable a police/shape on an interface you would use service-policy input/output (policy-map name)

Here is another good reference that may help QoS Fundamentals

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  • As I recall, 2960 switches can only do port-based QoS. You need the 3K or 4K series to do VLAN-based QoS.
    – Ron Maupin
    Aug 4 at 16:10

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