qos for voip

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Nov 20th, 2007
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Hi all, can anyone tell me firstly, when doing qos, if the packet has already been marked by the phone, do we still classify the traffic, then put it in the queue, and if its not marked what criteria is mainly used to match on for both on switches and routers ?

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Overall Rating: 3 (1 ratings)
yassine-m Tue, 11/20/2007 - 02:02
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If the IP phone has the ability to mark the packets the L2 switch don't need to mark the packet again but if not you should mark the 802.1p in the L2 switch and mark the DSCP field in the L3 switch.

Hope that it helps.


bvsnarayana03 Tue, 11/20/2007 - 02:28
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Ip phones generally mark the pkts originated by them.

You jst need to add "mls qos trust" comand under all interfaces connecting to ip phones.

carl_townshend Tue, 11/20/2007 - 02:39
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so if they are already marked and we want to prioritise them, what are the next steps in the switch and router ?

bvsnarayana03 Tue, 11/20/2007 - 06:27
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You need to create a class-map to match the voice-traffic. If u r using nbar, then match voice traffic as RTP.

Associate this to a policy-map, define the desired BW. Then apply the policy-map to interface.

If u r not using nbar, then match the traffic with acl & u may mark it prior to the traffic leaving the WAN.

You may refer this link for any configuration help:


pls rate if helped.


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