dgahm Wed, 03/07/2007 - 14:35
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Carl,

COS is layer 2 marking that happens in the trunk 802.1Q header. TOS is layer 3, and can be either 3 bits for IP precedence or 6 bits for DSCP.


Routers depend on the layer 3 TOS markings to classify traffic for QOS. DSCP is more granular than IPP and is best practice.


Older switches like 3548s were not TOS aware, forcing you to use COS only for QOS. Newer switches can be set to trust DSCP. There is an internal mapping from DSCP to COS that is important, but the default settings are usually OK for most switches.


Current best practice is to use DSCP in switches unless older switches forces you to use COS. In that case routers will need to have DSCP to COS mapping on interfaces connecting to trunk ports on switches.


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Dave

carl_townshend Wed, 03/07/2007 - 17:03
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so do i only need to set this on the first switch where the phone plugs in? and can you show me a sample config for a cisco and none cisco phone ?


cheers

dgahm Wed, 03/07/2007 - 17:54
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Carl,

QOS needs to be configured on every switch and router that will handle voice traffic. Here is a good document on understanding and configuring AutoQOS, which works pretty well, especially on switches. On routers it will at least give you a good starting point.


http://www.cisco.com/en/US/tech/tk543/tk759/technologies_white_paper09186a00801348bc.shtml


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Dave

dgahm Thu, 03/08/2007 - 09:59
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Carl,

TOS does not replace COS. New switches can use both. It is a matter of how you choose to use TOS and COS. When you configure AutoQOS for a Cisco phone the port will trust the COS setting from the phone, and use that for queueing traffic on egress. At the same time you can have a switch trunk port set to trust DSCP connected to a router.


Dave

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