Is there a need for QoS/CoS if we are under-utilizing bandwidth?

[J Sullivan]

We have a gig fiber up-link and only use about 200Mbps at any given time.  Is there a reason we would want to use QoS/CoS on our switches/routers?  Should mention that almost everything we do is cloud based, so internal host to host traffic is minimal.

[InayathUlla Sharieff]

Taking as a example Gig links then you have around 1024 Mbps then having 200 mbps should not be a issue.

Regards

Inayath

[Cisco Freak]

What if we want to prioritize some traffic out of that 200Mbps?

CF

[J Sullivan]

That's kind of where I was going.  We have VoiP and Video that we'd like to take priority, but I'm not sure if it would matter given the excess bandwidth we have.  

Would the switch or router buffers add to the latency at this point?  We have 3750 switches.

J

[InayathUlla Sharieff]

Voice / Video coming into picture then I would say that you use the QOS to avoid any impact to that traffic.

Regards

inayath

[Joseph W. Doherty]

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Posting

If you have congestion, sustained or transient, that's adverse to your traffic's service needs, and which can be mitigated by QoS, then you probably have a "reason" to use QoS.

Using about 200 Mbps, at any given time, on a single gig up-link, tells us very little about your network's end-to-end traffic handling, or your traffic's service needs.

[J Sullivan]

And what would be the best way to measure what those needs are?

[Joseph W. Doherty]

Depends on what you want to measure.

I know that seems a bit of a circleous answer but, again, it depends on the service needs of your traffic.

For example, consider we need to move a gig file across a 1 Mbps, but we also need to do so in a certain amount of time. If we need half a Mbps transfer rate, we might measure our transfer rate, to see if we're meeting our service need. Yet, are we measuring actual data transfer rate or wire bit rate?  With L2 and L3 overhead, and re-transmissions of data, a measured wire bit rate of half a Mbps might not meet our need to measure the effective data transfer rate.

For a different example, VoIP has a several needs that need to be met, and measured to insure they are.  There's end-to-end latency, jitter and lost data rates.  If you don't meet certain rates, your VoIP MoS tends to decay.