Why do we need QoS on LAN Switches

Quality of Service (QoS) on our LAN switches is often misunderstood. Now and then, people ask me why we need it since we have more than enough bandwidth. If we don’t have enough bandwidth it’s easier to add bandwidth than on our WAN links. If you use any real-time applications like Voice over IP on your networks then you should think about implementing QoS on your switches. Let me show you what could go wrong with our switches. Here’s an example:


Above, you see a computer connected to a switch with a gigabit interface. Between SW1 and SW2, there’s also a gigabit interface. Between SW2 and the server, there’s only a FastEthernet link. In the picture above, the computer is sending 400 Mbps of traffic toward the server. Of course, the FastEthernet link only has a bandwidth of 100 Mbps, so traffic will be dropped. Another example of traffic drops on our switches is something that might occur on Monday morning when all your users are logging in at the same time. Let me show you a picture:

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Forum Replies

  1. Hi Rene,
    Could you please do a short demo?
    I have 1x3560 and 2x3550, i have configured QoS (similar to the first topology) but i think it is not working well.

  2. Hi István,

    What would you like to see? We can do classification, marking, queuing, shaping and policing on the switches :slight_smile:


  3. Hi Rene,

    Do we have qos of service on LAN switches only?. Why not on a WAN and Router as well?.
    If we can have the same on Router, is it the same procedure as well as well.

  4. Hi Peter,

    QoS is applied everywhere, switches and routers. The configuration on routers is very different however…


  5. Hi Rene/Andrew

    Can you help me in understanding AutoQoS concept please ?


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