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Is QoS tools shape good for VoIP?


I have MPLS VPN scenario. For MPLS VPN at PE router connecting to hub, I've configured below QoS:

class-map match-any P1

match ip dscp ef

class-map match-any P3

match ip dscp default

class-map match-any P2

match ip dscp af31



policy-map QoS

class P1

priority percent 35

class P2

bandwidth percent 40


class P3

bandwidth percent 25


Since on the egree PE towords the internet, I'm using subinterface thus, the configuration is:

policy-map cust-qos

class class-default

shape average 256000

service-policy output QoS

interface FastEthernet1/0.8

encapsulation dot1q 8

ip address ...

service-policy output cust-qos

Since the internet access is 256K, I've configured shape average 256000.

My problem is that, during high congestion at internet access, I noticed that on cust-hub my VoIP SIP is having problem to ring to another VoIP SIP within the same hub. Is it because of the shape command ?

thanks in advanced


Re: Is QoS tools shape good for VoIP?

Hi Maher,

shape average 256000 is to much. The point is, that you have OSI layer2 overhead and layer2 keepalives and such. Lower it to f.e. 240000. You might need to "play" a little with the parameter, as the overhaed depends on average packet size.

Also make sure, that your SIP traffic is marked with af31. Just as a note: Cisco changed from marking voip signaling with af31 to cs3. Take a sniffer and check that proper markings are in place for voip signaling. Or just try

class-map match-any P2

match ip dscp af31 cs3

Hope this helps! Please rate all posts.

Regards, Martin

Community Member

Re: Is QoS tools shape good for VoIP?


Thanks for the advise. I mark and classify my ATA SIP on under ef. Would it be problem to it?

Regarding to "play" around with the parameter, I'm not that familiar. Is the any related documentation that I could read or perhaps some example?

Thanks in advance.


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