QOS Newbie Question

[JJ1]

I'm trying to understand some QoS statements which we have on our Catalyst 6500's and I was hoping someone might help me understand what some of the commands mean. If anyone could help me with the below questions, it would be much appreciated.

We have COS and QOS enabled and I'm told that these will be used to prioritise IP Phone traffic.

Question 1 - DSCP
I was wondering if someone could shed some light on what DSCP is? Who sets the DSCP bits and how does DSCP differ from the settings in the ToS field of the IP header? What is DSCP mapping?

Does this command enable DSCP inspection:
mls qos trust dscp

Question 2 - CoS
I understand that this is a value which the switches/routers add to every packet. Do they set CoS bits on a per-packet or per-vlan basis? Where in the IP packet is the CoS information held? Why would you use CoS when you can run QoS?

What does this command do:
mls qos cos 3 2


Question 3:
Could anyone shed some light on this command:
wrr-queue cos-map 2 1 3 4 5


Sorry for all the questions, but I couldn't find many other threads on QoS and I'm sure there are other people out there who would like to know about it.

Many thanks,

James.

 

[tschouten]

You can enter the mls qos trust command to set the trusted state of an interface. For example, you can set whether the packets arriving at an interface are trusted to carry the correct CoS, ToS, and DSCP classifications

dscp
(Optional) Specifies that the ToS bits in the incoming packets contain a DSCP value.

To define the default CoS value for an interface, use the mls qos cos command

Use the wrr-queue cos-map command to map CoS values to drop thresholds for a queue
wrr-queue cos-map queue-id threshold-id cos-1 ... cos-n

I know that probably isn't the answer you wanted to see, here is a good location for command information on the 6500

http://www.cisco.com/en/US/products...ommand_reference_chapter09186a0080161497.html