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New Member

Phone / Voice Quality isuses

Hi all,

I am experiencing phone/voice quality issues recently at a couple remote locations, users are reporting voice choppiness during conversation and phones reponding slow, like when they pickup the phone it takes a few seconds before they can hear the dialtone. And I noticed WAN traffic load was high when users reported the issue, so I am confident the issue is QoS related. Below is what I have configured on my WAN routers. Could someone help me clearify some questions or if there is anything wrong with my config ?

class-map match-any voice-signaling

match ip dscp cs3 af31

match protocol h323

match protocol mgcp

match protocol skinny

match access-group 101

class-map match-any Critical

match ip dscp ef

match protocol rtp audio

match ip precedence 5

class-map match-any Flash-Override

match ip precedence 4



policy-map Voice-Over-Wan

class Critical

priority percent 40

set dscp ef

class Flash-Override

bandwidth percent 30

class voice-signaling

bandwidth percent 5

class class-default


With the policy-map I have in place and with the "priority percent 40" for critical traffic. does this mean that I always have 40% of the link for critical traffic ? if yes, then what would cause the issue ?

I have T1 link at all remote sites and I noticed the issue happens when the link is at 90-95% utiliized.

thanks in advance !!!



Re: Phone / Voice Quality isuses

It means you set LLQ for voice traffic which is matched as RTP audio or dscp EF or precedence 5. Means this traffic will have priority over any other traffic and will take MAXIMUM 40% of your link. Anything over this number will be dropped means choppy voice.

I would check bandwidth configuration on your link as first thing. Maybe other unwanted traffic is marked as EF or precedence 5 before reach your router. This could cause some issues for your voice traffic.

Last thing could be more than 40% bandwidth usage for phone calls.

Use NBAR to check your traffic patterns on this link it can help in early stage of defining problem.

rate if this help

Re: Phone / Voice Quality isuses

Hi Danny,

You may want to try increasing your priority percentage to see if that helps. If you're dropping packets a 'show interface' and 'show policy-map interface' should help you figure that out.

What is also helpful on your T1 circuits is to change the tx-ring-limit to 2:

interface serial0

tx-ring-limit 2

What happens is that QoS is only applied to software queued packets. If you have a large tx-ring, you may have packets sitting in the ring holding up your priority packets. By reducing this, you put your packets in software and allow your router to more articulately determine which packets go out first.


New Member

Re: Phone / Voice Quality isuses

Thanks Nick !!! I appreciate your response !!! I will try the "tx-ring-limit 2" command see if that helps.

thanks again !!


New Member

Re: Phone / Voice Quality isuses

Hi Nick,

One quick question. Does the size of the tx-ring depend on the type of WAN circuit ? for example, what would be the size of the tx-ring for DS3 circuit or bonded T1s ?

Thank you very much !!!


Hall of Fame Super Gold

Re: Phone / Voice Quality isuses

On all cisco platforms, tx-ring-limit is set to an optimal value in presence of priority class on an interface.

If you google on the subject you may find how to check it, likely in show controllers.