QOS on vlan interfaces?

Posted on 2011-10-06
Last Modified: 2012-05-12
I have never seen QOS applied to a vlan interfaces but the thought crossed my mind why or why not use QOS on a vlan interface?

I looked at the commands under a vlan interface and there is not any QOS commands available so I suppose there is no way to configure QOS.

I understand that basically a vlan interface is a virtual interface and not a physical one.

Is that why? But aren't the buffers queues for the interfaces kind of virtual also?

Maybe the question is contrast and compare real interfaces vs virtual interfaces?
Question by:Dragon0x40
    LVL 26

    Expert Comment

    You can apply qos to vlan interfaces. What device are you on?
    LVL 26

    Accepted Solution

    You don't have the service-policy command available under your vlan interfaces?

    Author Comment

    you are correct there is a service-policy command available.

    If traffic tagged with EF46 crosses a vlan interface without any qos configured will the tag get stripped?
    LVL 6

    Expert Comment

    Even if no QoS is configured, the tag should not get stripped as long as it is an IP packet cause DSCP will always exist in the packet header.

    Author Comment

    My understanding is that if qos is turned off globally on a switch then the original tags remain, so by default the switches will not strip the qos tags.

    If a switch has qos turned on then any interface that is not set to trust dscp values will strip the qos tags?

    if my switch has qos turned on and trust dscp on all interfaces except the vlan interfaces what keeps the vlan interface from stripping the tag?
    LVL 6

    Expert Comment

    that's true. if QoS is enabled without settings, the switch will strip the DSCP from the packets.
    So for the vlan interface, it will really strip it in that case.

    Author Comment

    To keep the tag on don't you use the "mls qos trust dscp" or "qos trust dscp"?
     I don't see an option to trust any tags under the vlan interface only the service-policy command?
    Does service-policy have to be used or the qos tag gets stripped?

    (config)#int vlan XX
    Interface configuration commands:
      appletalk       Appletalk interface subcommands
      arp             Set arp type (arpa, probe, snap) or timeout
      backup          Modify backup parameters
      bandwidth       Set bandwidth informational parameter
      bgp-policy      Apply policy propagated by bgp community string
      carrier-delay   Specify delay for interface transitions
      cdp             CDP interface subcommands
      clns            CLNS interface subcommands
      cts             Configure Cisco Trusted Security
      dampening       Enable event dampening
      default         Set a command to its defaults
      delay           Specify interface throughput delay
      description     Interface specific description
      eigrp           EIGRP interface specific commands
      energywise      EnergyWise interface configurations
      eou             EAPoUDP Interface Configuration Commands
      ethernet        Ethernet interface parameters
      exit            Exit from interface configuration mode
      glbp            Gateway Load Balancing Protocol interface commands
      help            Description of the interactive help system
      ip              Interface Internet Protocol config commands
      ipv6            IPv6 interface subcommands
      ipx             Novell/IPX interface subcommands
      isis            IS-IS commands
      iso-igrp        ISO-IGRP interface subcommands
      load-interval   Specify interval for load calculation for an interface
      logging         Configure logging for interface
      loopback        Configure internal loopback on an interface
      macro           Command macro
      mtu             Set the interface Maximum Transmission Unit (MTU)
      nmsp            NMSP interface configuration
      no              Negate a command or set its defaults
      ntp             Configure NTP
      private-vlan    Configure private VLAN SVI interface settings
      qos             QoS configuration
      service-policy  Configure QoS Service Policy
      shutdown        Shutdown the selected interface
      snmp            Modify SNMP interface parameters
      source          Get config from another source
      spanning-tree   Spanning Tree Subsystem
      standby         HSRP interface configuration commands
      timeout         Define timeout values for this interface
      vrrp            VRRP Interface configuration commands
      vtp             Enable VTP on this interface
    LVL 26

    Assisted Solution

    You don't trust tags on vlan interfaces only on the physical ports. That is why you don't see the option. The vlan interface is where you will match the traffic and act upon it.

    Author Comment

    I don't see a way to trust cos or dscp on a vlan interface so I am beginning to think that the vlan interface is not going to modify the QOS tags. Unless the policy-map referenced by a service-policy applied to the vlan interface changes the QOS tags?

    physical interface
    (config-if)#mls qos trust ?
      cos            cos keyword
      device         trusted device class
      dscp           dscp keyword
      ip-precedence  ip-precedence keyword

    vlan interface
    (config-if)#service-policy ?
      input   Assign policy-map to the input of an interface
      output  Assign policy-map to the output of an interface
    LVL 26

    Assisted Solution

    You are correct. The policy on the vlan interface applies to any traffic entering or exiting the vlan, not a specific port. Whereas the policy on the port will affect the port only. Yes, with the options you present, the vlan interface will not modify the qos marking unless the policy map matches then changes the marking.

    Write Comment

    Please enter a first name

    Please enter a last name

    We will never share this with anyone.

    Featured Post

    Enabling OSINT in Activity Based Intelligence

    Activity based intelligence (ABI) requires access to all available sources of data. Recorded Future allows analysts to observe structured data on the open, deep, and dark web.

    Meet the world's only “Transparent Cloud™” from Superb Internet Corporation. Now, you can experience firsthand a cloud platform that consistently outperforms Amazon Web Services (AWS), IBM’s Softlayer, and Microsoft’s Azure when it comes to CPU and …
    I recently attended Cisco Live! in Las Vegas, a conference that boasted over 28,000 techies in attendance, and a week of hands-on learning hosted by a solid partner with which Concerto goes to market.  Every year, Cisco displays cutting-edge technol…
    Here's a very brief overview of the methods PRTG Network Monitor ( offers for monitoring bandwidth, to help you decide which methods you´d like to investigate in more detail.  The methods are covered in more detail in o…
    This video gives you a great overview about bandwidth monitoring with SNMP and WMI with our network monitoring solution PRTG Network Monitor ( If you're looking for how to monitor bandwidth using netflow or packet s…

    760 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    9 Experts available now in Live!

    Get 1:1 Help Now