How to keep ToS in a 2950/2955 switch

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Nov 14th, 2007
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I have a device which sends voice over IP packets marked with ToS only (ToS=3).

All packets pass through a 2955 switch.

From the configuration guide i see that the switch supports only CoS and DSCP.

How can I forward the Voice packets and still keep the ToS field?

At the edge of my LAN I have a modem/router that can read only the ToS field and I am concerned that the modem wont be able to prefer the voice over the data.

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colin.mccrory Wed, 11/14/2007 - 15:26
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You can configure the switch port to trust a setting in the IP header (TOS or DSCP) using the following commands.

1. Under the interface, configure:

auto qos voip trust

This will setup switch for all relevant QOS settings. You will notice all the QOS settings now appear under the global config.

2. Go back under the interface and configure:

mls qos trust dscp.

3. Do this for all ports required making sure you also enable QOS on any uplinks.



interface FastEthernet0/2

mls qos trust dscp

auto qos voip trust

spanning-tree portfast


This will trust your TOS settings and present them to the router.

Pavel Bykov Wed, 11/14/2007 - 16:57
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Please understand: ToS is field in IP packet, and is now a DSCP.

Therefore (field wise) ToS = DSCP and on the switch you should allow TRUST DSCP, which will enable passing of ToS field unchanged.

Hope this helps.

danny4569 Thu, 11/15/2007 - 04:41
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I will check it.

Is'nt ToS bits (3 bits) different from DSCP bits (8 bits)?


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