Hardware vs. Software QoS

Answered Question
Feb 1st, 2008

I had this come up in an interview setting where the interviews asked me to describe in detail the differences between hardware and software QoS. I know how to configure some of the QoS tools with MQC, but I don't know what was meant by this.

Can someone explain to me what they may have been looking for?



I have this problem too.
0 votes
Correct Answer by Edison Ortiz about 8 years 11 months ago

> this kind of hardware already built into the Supervisor?


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Overall Rating: 5 (1 ratings)
Jon Marshall Fri, 02/01/2008 - 09:03

Hi Jim

Very generally speaking routers do QOS in software whereas switches do QOS in hardware. What they were probably looking for, although who knows what interviewers are looking for with some questions :), was that QOS in software uses a standard command set across all routers MQC, whereas switches because it is ASIC based do not ie. configuring QOS on a 6500 you do not use the same commands as on a 4500 switch. There are some similiarities but also quite a few differences.

QOS on Catalyst switches can be a bit of a bear !


Edison Ortiz Fri, 02/01/2008 - 09:23

The difference is, when you implement QoS and it runs in software, all cycles are punted to the CPU so the switch may run slower while if you implement QoS and all the features are supported in hardware, the CPU is not used at all. The switch itself has built-in hardware feature card or chip that was designed specifically to handle QoS.




Danilo Dy Fri, 02/01/2008 - 09:34

This kind of question in an interview is funny and weak.

There are multiple possible answer to this question, who knows what the interviewer think is the right answer - he may not even know that your answer is right if your answer is not what he is expecting.

Interviewer should ask technical questions that there is only one possible answer, this is to avoid confusion and lengthy interview. The first question should have an answer of either "Yes" or "No", if the applicant answered it correctly, the interviewer will then ask the applicant to explain it briefly.

It's like telling the interviewer "I have 5 possible answers to your question, choose A, B, C, D, or none of the above" :)

Jon Marshall Fri, 02/01/2008 - 09:40

Hi Dandy

Think me and Edison proved your point ie. i addressed it from a configuration point of view and Edison from a performance point of view.

I think these sort of questions can be useful in intterviews though. Sometimes it's important to guage how well a person understands the general concepts rather than just do you remember this command etc.

Just my thoughts.


Edison Ortiz Fri, 02/01/2008 - 09:52

It also depends on the position you are applying for. I usually ask all kind of questions, from really basic to ones not many people can answer.

I don't expect an answer on every single question and I don't hold that against the person applying for the job but I want to see how the person really thinks. Answering "I don't know" it's better than coming up with something that makes no sense at all.

jim_berlow Fri, 02/01/2008 - 10:43

I think you guys have hit right on the nail. So does hardware-based QoS utilize upgrade modules (for instance on a Sup 720) or is this kind of hardware already built into the Supervisor?

The interviewer asked a lot of subjective questions where there were many directions a person could take. I think he was using this line of questioning to see if I could explain concepts, communicate clearly and not "trip" on myself explaining things.

To give you an idea, another question was "Tell me everything you know about IPSec VPN's". Obviously, there is a whole mountain of information on that topic and easy for a person to fall trap to how they word their answer.

Thank you,


Correct Answer
Edison Ortiz Fri, 02/01/2008 - 12:25

> this kind of hardware already built into the Supervisor?



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