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Cisco phones default voip QoS tagging over a vpn

Posted on 2012-03-20
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I have a small satellite office that i am trying to run a couple of Cisco phones over a VPN.  I know this isn't the most desirable way and calls over vpn are prone to jitter.  I can still see some benefit in adding QoS to the source and destination side of the tunnel especially if the source is maxing out its connection.

I have been told that Cisco phones use "cost" as default for QoS.
(1) does anyone have a clarify explanation on this? and is there a way to utilize this in a non Cisco environment?

(2) do Cisco phone default tag there traffic using any of the DSCP tags? such as AF41,42 or EF?

(3) Any advice in general would be appreciated here on this.

Regards,

-Steve-
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Question by:Steve262
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by:mat1458
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>I have been told that Cisco phones use "cost" as default for QoS.
>(1) does anyone have a clarify explanation on this? and is there a way to utilize this in a non >Cisco environment?
Cisco phone mark COS (not cost, meaning Class of Service or in other words Layer 2 priority bits in the 802.1 header) and DSCP.

>(2) do Cisco phone default tag there traffic using any of the DSCP tags? such as AF41,42 >or EF?
Cisco phones mark DSCP EF for bearer traffic and depending on your UCM software the signaling traffic as CS3 or AF31. Read the documentation of your Call Management software for more detailed information.

>(3) Any advice in general would be appreciated here on this.
Not that it necessarily helps but if you can do some qos preclassification on your VPN device some accidential QoS capability in the network in between can be used. Use at least a priority queue on your VPN device/edge router for the voice traffic.
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TimotiSt earned 1000 total points
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The layer 2 COS marking won't be forwarded on the VPN tunnel, since it's not ethernet.

The QOS design would depend on the connection type/medium, but I'd set up priority handling for voice traffic going through the VPN, and maybe QoS on the incoming packets by your ISP, prioritizing IP packets from the other side of the VPN (the outside address of your head office).

Depending on your switches, you may want to remark COS on the ethernet frames exiting the router towards your local networks. For example, Cisco 3500XL switches can't do QoS based on DSCP, only on COS.

Tamas
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