Does voice priority queue hold % of bandwidth even without any voice traffic?

[jkeeffe]

I understand that QoS queueing doesn't kick in until an interface gets congested. However, once congestion is occurring, does the total size of the priority queue get held for priority traffic even though no priority traffic, or very little, is traversing the interface?

For example, on a 100MB router interface I have applied a service policy that guarantees 30% of the 100MB for priority (EF) traffic (voice and video) - the rest is class-default. If there is congestion on the interface, say someone is pulling a huge FTP file, and there are only a few voice calls going on equalling 1mbps, does the entire 30% priority kick in, thereby leaving only 70% remaining bandwidth for the FTP transfer? Or is the remaining bandwidth for class-default inversely proportional to the amount of actual voice traffic at any given point - up to the 30% priority queue quarantee?

I'm curious if class-default, during congestion, is left with the same amount of bandwidth remaining (70%) whether there is priority traffic or not.

[Steven Griffin]

The short answer is that it depends on the device, version of code on the device, and in some cases the IOS command used.  Some devices/commands enforce strict priority and do carve out the bandwidth while others will let excess priority queue bandwidth be used for non-priority traffic to fill up the timeslot.

You'll need to consult the QoS guide for the IOS version you are using to see if your device hardware supports reallocating unused priority queue bandwidth or has a CPU based QoS mechanism that allows it.

-Steven

[Chris Deren]

Uscitizen,

Which devices/IOS code are you referring to that work this way?  My understanding is that all QoS capable devices including routers and switches have all bandwidth available to all traffic unless strick traffic shaping or policing is configured.  QoS is only useful when there is congestion on the device/link, if there is no congestion the bandwidth is available to other traffic classes.  Perhaps I always misunderstood it, so I would be very curious to see otherwise.

HTH,

Chris