QOS in relation to high bandwidth utilization

[Dustin Burke]

Hi all,

I just need to confirm if a router/switch does NOT have high utilization, does QOS still kick in? I have a strange situation with one of my devices showing drops, but the bandwidth is no where near saturation. However I do see drops for one of the the queue for QOS.

Anyone have any suggestions?

[Reza Sharifi · Accepted solution]

Hi,

QOS will kick in only when there is congestion on the physical interface.

Do you see the drops all the time for the queue or it was just one time?  If it was one time than maybe you have had congestion at some point.  Clear the counter and watch the interface for a period of time.

HTH

[Dustin Burke]

Hi Reza,

There are 3 subinterfaces for my WAN and ive noticed 2 of the 3 take some drops. However its taking drops in a higher queue while the default class do not have any drops. I did see on NBAR some high amount of bits used within 30 seconds. Its a constant increase in drops. I just thought since it was a high AF value it would drop the lower (class default) rather than the more critical data. NBAR is activated so I can see the high traffic usage for the protocols, but maybe I should review the policy again.

I guess my question was have you ever seen QOS kick in on an interface that doesnt have high utilization.

Thank you for your help!

Normally I am pretty good with QOS but I just thought this was strange.

[Joseph W. Doherty]

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Posting

As Reza correctly noted, normally QoS only engages when an interface becomes congested, i.e. packets are queued.

Utilization is measured over time, and it's often possible a very short duration burst can overflow a queue (causing drops) yet the utilization is "low".  (Conversely high utilization doesn't always lead to drops.)

Often in cases like yours, low utilization that show drops, drops can often be mitigated by increasing queuing resources (to better handle a short term burst).