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Does the priority queue always work?


I have a 8Mbp of wan link which sometime gets saturated and I have shaped average this to 8Mbps but i am running vocie on this WAN link and have defined priority for voice with 850kbps under voice class. My question is when the link is not fully utilized, Will the packets from priority queue are always dequeued first as compared to packets sent from from other queus or will the QoS will not do anything here since the link utilization is lot less than what is sepecified in shape average. I am asking this to confirm if the priority queue always help to overcome the issue of jitter if either the link is saturated or not?


VIP Purple

From the many QoS-tools

From the many QoS-tools available, there are some that work always and some that only get active under congestion. The priority-queuing configured in your LLQ-class is only active when there is a congestion. But it's also only needed in that case. If there is no congestion, then your voice-packets don't get an extra delay because they can be sent out immediately.

More on this topic is available in the configuration guides:

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You describe PQ and shaping, but the former is usually a part of doing QoS on L2/L3 switches, and the latter on routers.  What device(s) and their IOS versions and the WAN media are you working with?


On "routers", interfaces generally have FIFO tx-rings, only when they overflow, are packets placed in CBWFQ queues.  Within CBWFQ, LLQ will be dequeued first, but such packets might have already been queued behind other non-LLQ traffic within the interface tx-ring.  (NB: for routers, with tx-rings, when supporting VoIP, you may want to minimize the size of the tx-ring.)


Shapers, in my experience, are "interesting".  First, I believe many shapers don't account for L2 overhead, but provider CIRs often do.  So unless you shape slower than the nomimal CIR rate, you can send faster than the available bandwidth.  (Often I've found shaping 10 to 15% slower allows for average L2 overhead.)


Second, shapers work on averages over time intervals.  For VoIP, you'll often want to insure the shaper is using a small Tc, otherwise it will allow FIFO bursts.  (I've found a Tc of 10ms seems to support VoIP fairly well.)


Third, I suspect some shapers might have their own queues between the interface and the defined policy queues.  If they do, unknown what their queuing organization is or their supported queuing depths.  If this is the case, makes it difficult to engineer QoS.


Whenever possible, I've found it beneficial to work to avoid using shapers especially for timing sensitive traffic, like VoIP.  In your case, I would suggest, if possible, obtaining 10 Mbps of WAN bandwidth and somewhere passing the traffic through a physical 10 Mbps interface, with a QoS policy.


But to more directly answer your question, PQ (or LQ) will dequeue its packets next compared to other "peer" queues.  This should always help VoIP for delay and jitter, but there's more involved whether this is necessary and/or whether it's helpful enough when necessary.


You ask about when a link is saturated, but a link is 100% saturated everytime a packet is being transmitted.  Often link usage is represented in percentages of usage of possible maximum transmission rate over some time period, but when it comes to QoS, 100% utilization might be just fine while 1% utilization is not.  Much, much more information, about your situation, might be needed to offer truly constructive recommendations.