QoS for VoIP

Unanswered Question
Mar 10th, 2007
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I have 8 sites connected together over a full mesh MPLS.

One site has international connectivity where my VoIP gateway is located. It has 2 PRI lines terminated onto it.

Each site has a number of VoIP phones and they mainly use it for international calling. During Peak traffic the VoIP quality degrades.

Qos is configured on the LAN by trusting the DSCP values on the trunk. On the routers connecting to MPLS, the following QoS has been configured

class-map match-all voip

match ip precedence 5

class-map match-all signalling

match ip precedence 3

policy-map VOIP

class M6logserver

police 8000 conform-action transmit exceed-action drop

class voip

priority 1024

class signalling

priority 200

interface FastEthernet0/1

service-policy output VOIP

I do not see any drops on the policy-map interface

Is there a better way to configure the QoS

thanks in advance

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dgahm Sat, 03/10/2007 - 22:25
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Does your MPLS provider apply QOS to protect your voice traffic? If your routers are not dropping packets then it must be happening in the MPLS cloud.

I am wondering about 1024 for 46 voice channels. How full do the PRIs get? Assuming G729 you should allocate at least 26kb per call or 1196kb if there is any chance of filling both PRIs. Your signaling class does not need to be priority queued, just use bandwidth 200.

Another possibility is that the DSCP markings are getting overwritten to 0s before getting to your routers. Does the priority queue bandwidth (show policy-map interface) at site 4 match up with the number of active calls (sh voice call summ)on the voice gateway router? If you set the load interval to 30 it will track more closely. A Sniffer would also confirm the DSCP. It is pretty easy to misconfigure a switch and overwrite the DSCP values.


mtechnology Sun, 03/18/2007 - 06:32
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The MPLS provider is doing QoS on his cloud. Thats what they say.

On the LAN switches i am just trusting the DSCP on the lan switches (no marking or classification)

Sometimes during peak utilization, the latency from site1 to site 4 exceeds to 200ms and hence all international calls suffer.

Will it help if i create a seperate vlan for voice and mark the DSCP at the access and configure queuing.

paolo bevilacqua Sun, 03/18/2007 - 09:15
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Before configuring anything, check if you can have the voice traffic be marked EF at the source and sent unchanged to the provider.

You can also configure a SLA probe/ responder sending UDP / EF traffic and measure delay/jitter over time.

Once you have proof that the problem is in the cloud, call the provider and complain to them.


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