# Introduction to QoS (Quality of Service) **URL:** \ **Category:** Lessons Discussion\ **Created:** [May 3, 2017, 11:38am UTC](https://forum.networklessons.com/t/introduction-to-qos-quality-of-service/1703 "2017-05-03T11:38:29Z")\ **Posts on this page:** 1\ **Showing post:** 74
**Author:** ![lagapidis](https://cdn-forum.networklessons.com/user_avatar/forum.networklessons.com/lagapidis/32/4949_2.png) [@lagapidis](https://forum.networklessons.com/u/lagapidis)\ **Post date:** [December 5, 2024, 6:05am UTC](https://forum.networklessons.com/t/introduction-to-qos-quality-of-service/1703/74 "2024-12-05T06:05:13Z")
Hello David Nice to see you after a while! > [@davidilles](#): > > Or in other words, the configured traffic prioritization and queuing and such doesn’t happen until we have congestion, correct? Yes that is correct. Queuing only takes place when there is congestion. Think about it this way. If you go to a bank, and there is no lineup, you are served immediately. There is no queue to manage!! So there is no meaning to employing CBWFQ/LLQ or any other queue management algorithm if there is no queue! > [@davidilles](#): > > My second question is, if we connect two devices using a 1 gigabit link, it’s not exactly right to say that congestion will occur once we go over 1 gigabit, right? Because we should also account the devices’ performance into this. Even if the link is 1000 mbits, congestion can occur earlier if the device’s performance can only handle, let’s say, 250 mbits, right? Ah yes. The question here is “what is congestion?” Is it when the port itself reaches its capacity or does congestion also occur when the CPU and memory resources of the network device are overwhelmed? Or is it when the [backplane](https://notes.networklessons.com/switching-backplane) of the device has reached capacity, even if the specific port in question has not? QoS mechanisms are primarily designed to deal directly with port-level congestion. When the port’s egress queue (or ingress queue, depending on the device and QoS design) fills up, QoS mechanisms like traffic shaping, policing, scheduling, and queue management kick in. So to answer your question, congestion, as it pertains to QoS, is most commonly tied to **oversubscription of physical or logical interfaces and queue overflow**. CPU, memory, or backplane congestion can also exist but are not the typical focus of QoS mechanisms. As always, it’s a pleasure to answer your questions… I hope this has been helpful! Laz --- _[View the full topic](https://forum.networklessons.com/t/introduction-to-qos-quality-of-service/1703)._