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:

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Forum Replies

  1. Hi Rene/Andrew

    Can you help me in understanding AutoQoS concept please ?


  2. Hi Abhishek,

    Let me jump in to give you a brief explanation of Cisco AutoQos.

    Configuring QOS can be a challenge for engineers who aren’t familiar with this topic, but leaving the network without a QOS configuration can cause a lot troubles especially with the bandwidth-hungry applications and the real-time traffic such as VOIP and Video which can partially and/or fully not work if QOS isn’t configured, in addition of problems like latency, jitter, and packet drops.

    For this reason, Cisco IOS Software offers a portfolio of QoS features which is enable by a or f

    ... Continue reading in our forum

  3. Hello Sims

    This is a very good question that doesn’t have a clear cut answer.

    In the hierarchical model, the core is performing high speed routing and switching to get things from place to place, the distribution layer is applying policies and filtering, and the access layer is providing you with port density and user connectivity. Based on this, and based on most textbooks and CCNP study material, QoS is said to be most commonly applied at the distribution layer.

    However, Layer 2 switches at the access layer can perform both marking and queuing and in general

    ... Continue reading in our forum

  4. Hello Zouhair

    In such a scenario, I assume you are focusing on the QoS mechanisms for the voice packets delivered by the IP phones and softphones. Classification and marking of voice packets should be done at the devices themselves. A more detailed explanation can be found in the Classification and Marking section of the following lesson:


    Once voice packets are classified and marked, they must be acted upon by QoS mechanisms. Such

    ... Continue reading in our forum

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