Why do we need QoS on LAN Switches

Quality of Service (QoS) on our LAN switches is often misunderstood. Every 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 towards 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:

Digiprove sealCopyright protected by Digiprove © 2013-2016 Rene Molenaar

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You’ve Ever Spent on Your Cisco Career!
  • Full Access to our 745 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

551 Sign Ups in the last 30 days

100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

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 ?


7 more replies! Ask a question or join the discussion by visiting our Community Forum