cos and qos

carl_townshend · ‎03-07-2007

Hi all, can anyone tell me if we need to use both when setting up a voice network, i believe cos is on switch and qos it on router? can we just use cos and it easy to configure

dgahm · ‎03-07-2007

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.

Please rate helpful posts.

Dave

carl_townshend · ‎03-07-2007

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 · ‎03-07-2007

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

Please rate helpful posts.

Dave

carl_townshend · ‎03-08-2007

so does the TOS replace COS on switches ?

dgahm · ‎03-08-2007

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