QoS question

Unanswered Question
Jul 6th, 2008

If LLQ is configured using "priority" to handle only certain number of calls (say 24) what happens to the 25th caller? Does it get dropped or does it allowed to establish with lower priority? I believe the former is true.

BTW..I am talking about VOIP traffic without CAC being configured.

Any thoughts!

I have this problem too.
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andrew.butterworth Sun, 07/06/2008 - 05:19

It depends on how the policy is configured and whether there is congestion. If the class is configured with the police action 'drop' then obviously traffic at a higher rate than the policy allows will be dropped. If a drop action isn't configured then it will be forwarded unless there is congestion.

This explains it better than I could:




fawad.alam Sun, 07/06/2008 - 05:36

Andy, your right that it will depend on whether or not there is congestion in network. LLQ does metering and utilizes a built-in policing mechanism that kicks in only when there is congestion in network.

So for my question, I am still not sure what will happen in the following situation:

1. LLQ is configured to allow 24 calls.

2. Network is experiencing congestion.

3. LLQ is not configured qith "policing" but ofcourse it will have the built-in policing mechanism working in this scenario since network is experiencing congestion.

4. WHAT will happen to the 25th caller?


Jon Marshall Sun, 07/06/2008 - 05:53


My understanding is that if you are using Diffserv for your QOS the 25th caller will be allowed onto the network. If the network is then congested in theory all 25 calls can be degraded because packets may be randomly dropped from any of the VOIP streams.

If you want to ensure that the 25th call is not allowed on the network if there is not enough bandwidth for it you need to look at Intserv to reserve bandwidth prior to allowing the traffic onto the network.


fawad.alam Sun, 07/06/2008 - 06:36

Hi Jon,

I still think that 25th call will be dropped, and if not, why is there a built-in policing mechanism in LLQ.

Also, there will be no random drops as LLQ has following restrictions:

LLQ does not allow random-detect, queue-limit, and bandwidth commands. LLQ uses tail-drop as congestion-avoidance mechanism.


Jon Marshall Sun, 07/06/2008 - 07:30


Random may not have been the best word to use. What i was trying to get across was that if you prioritise VOIP traffic then all VOIP traffic is treated equally within that class ie. packets from one call are no different from packets from another call.

As for the 25th call being dropped. It may and it may not but that is the whole point. Lets say you have

H1 - L2 switch -> R1 -> R2 -> L2 switch - H2

H1 is making a call to H2 (and this is the 25th call). If the congestion is between R1 & R2 then the VOIP packets will be allowed onto the network at the L2 switch and then will reach R1. Packets from this call may well be dropped, but then so may packets from any of the other calls and it depends on which order they are in the queue.

That is why Diffserv is described as per-hop rather than end-to-end. Each hop treats the packets in isolation as such.

There is an inbuilt policing mechanism for LLQ to ensure there is always available bandwidth for other priorities. The problem with the original priority queue implementation was that the priority queue was always serviced before other queues but this could mean other queues were never serviced if the priority queue kept the router busy.

The in-built LLQ policing mechanism ensures that the other queues will still be serviced in periods of congestion.


Edison Ortiz Sun, 07/06/2008 - 07:56


LLQ will police the queue based on the configured bandwidth. If you allocate enough bandwidth for 24 calls, those calls won't experience drops (again, this configuration must be consistent in each hop on the network for end-to-end QoS).

The 25th call will be treated as a normal packet and it will be dropped or experience jitter/delay during congestion as it will not be de-queued first.

There are also options within the priority command to drop excess traffic or change its DSCP setting and forward the packet.



Jon Marshall Sun, 07/06/2008 - 08:10


Agreed. Think i'm having a few problems explaining myself or i'm just talking rubbish !!

"The 25th call will be treated as a normal packet and it will be dropped or experience jitter/delay during congestion as it will not be de-queued first."

I think what i was getting at is what are the chances of the packets arriving in the exact order that it would be the 25th call's packets that would be dropped. Yes VOIP packets would be dropped but due to the constant fluctuations in network traffic it just seemed to me that if you wanted to guarantee it was the 25th calls packets then that is what Intserv/RSVP was intended to do.

Perhaps i'm just being a bit too literal :-)


Edison Ortiz Sun, 07/06/2008 - 08:45

Got it.

In other words, the 25th call isn't marked as the 25th call on the end-to-end QoS so a random conversation 1-24th may get dropped due to the priority being exceeded.

You aren't being literal, you are dead-on :)




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