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: