2950 DSCP/QOS behavior

Answered Question
Aug 5th, 2010

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?



Correct Answer by Collin Clark about 6 years 6 months ago

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.


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Overall Rating: 4.5 (2 ratings)
Collin Clark Thu, 08/05/2010 - 08:19


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?



billmatthews Fri, 08/06/2010 - 12:42

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

Kristoff.mace Wed, 02/23/2011 - 10:22


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.



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