2950 DSCP/QOS behavior

[billmatthews]

We have some 2960s in our environment, and we have the switchport and trunk ports configured with

mls qos trust cos

That works fine, and DSCP tags are being passed as-is.

However we had the same command issued on our 2950s, and DSCP was being rewritten/stripped.  I found this document http://www.cisco.com/en/US/tech/tk389/tk813/technologies_q_and_a_item09186a00804976d3.shtml#qa10

Based on that doc I changed it to mls qos trust cos pass-through dscp and the tags are now being passed as-is.

My question is, is the 2950 actually prioritizing packets (honoring DSCP) or is it just passing the tags?

Thanks

Bill

[Collin Clark]

Bill,

You can configure it to. Take a look at the link you provided and go down two questions, you should find your answer.

Q. What kind of output scheduling do the Catalyst 2950 series switches provide?

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[billmatthews]

Thanks Colin.  +5

I read that question, and it certainly seems to provide the answer, but I was wondering if I could get more clarification on what these commands do? Is each wrr-queue creating a forwarding queue?  And then cos-map 4 5 is for expedited forwarding?  Our ISP said to just make sure that we have an expedited forwarding queue setup.   We just used auto qos voip and it set this up:

wrr-queue bandwidth 10 20 70 1
wrr-queue cos-map 1 0 1
wrr-queue cos-map 2 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
mls qos map cos-dscp 0 8 16 26 32 46 48 56

Assuming our voice is being marked as cos 4, does that give us what we need?

Please point me to some intro cos reading if that would be easier.  Thanks

[Collin Clark · Accepted solution]

It has been a long time since I've worked with the 2950's and I don't remember which is the priority queue. I would assume 4. Here is a link to the SRND for QoS. It's an excellent guide and the official Cisco QoS training class uses this as the guide book.

http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book.html

[Kristoff.mace]

Hello,

To define a strict priority queue on 2950 (expedite queue) you have to configure this:

wrr-queue bandwitch 10 20 70 0

0 means that queue 4 is an expedite queue.

Be carreful with the version (SI or EI) of the 2950.

Regards.