Enabling "auto qos trust dscp" across environment

[esa_fresa]

Hi All - I'm hoping to verify my understanding of QoS is correct.

I'd like to enable QoS for media and call signaling as much as possible throughout our environment. Our call center connects to a 2960x stack attached to two 2911 ISR's with MPLS connections up to our datacenter where our UC cluster is housed. 

Currently the plan is to enable QoS on all access ports and uplink ports with the "auto qos trust dscp" command. We're going to use Group Policy to tell our end devices (running Jabber) to mark dscp. So my questions are...

1. Is enabling "auto qos trust dscp" across all network gear the right way to do this?

2. Should we use dscp all the way, or is there a reason to convert dscp to CoS?

3. Any "gotchas" or things we should watch out for?

Catalyst 2960-X Switch QoS Configuration Guide, Cisco IOS Release 15.0(2)EX

Cisco Jabber 10.6 Deployment and Installation Guide

[Packet Herder7]

"Auto" under Cisco is great when you are working with Cisco products/services so go for it based on their documentation.

In this case, I believe that configuring it across the board on the expected traffic path will serve you well. Obviously...test it out and see what happens. In most cases, you do not have to worry about the dscp values but under a stressed network (a lot of traffic, etc), it might be prudent to make sure that those values guarantee your critical applications' traffic.

[Joseph W. Doherty]

#2 avoid using L2 Cos, use L3 DSCP whenever possible.
#1 and #3 Well, usually Cisco's auto QoS protects VoIP type traffic. For the rest, it depends whether their QoS model is suitable for your needs. If you're going to do more than protect VoIP over BE traffic, you should review the auto QoS settings and determine if it's what you want.

It's easy to make things worst with poor QoS policies. Don't assume Cisco's auto QoS might not make things worst - much depends on your actual traffic.

[esa_fresa]

Thanks Joseph. #2 Can you elaborate on why we should use DSCP over CoS? 

[Joseph W. Doherty]

CoS requires VLAN tagging, on the frame, and is lost at every L3 hop (which would require resetting it).

DSCP can go end-to-end. Also DSCP offers a much larger code space.

[esa_fresa]

Okay, that makes sense.

So we only have VoIP traffic requiring QoS (Cisco IP Phones, Jabber for Windows, Webex, Telepresence units such as MX800). We don't have any other critical or timesensitive traffic. I did review the auto-QoS documentation and it looks like it will do what I need, I was hoping for a second opinion on here though. From what Packetherder said I'm feeling good about it. Do you have the same thoughts?

[Joseph W. Doherty]

Over the years, I've done quite a bit of QoS and haven't used the auto variants. This due to a couple of reasons. First, auto QoS tends to change from time to time (fortunately not to frequently), so your QoS policy might change as Cisco changes it. Second, depending on the particular "flavor" of auto QoS a device implements, I've rarely found it to do what I want QoS to do (but then I rarely completely agree with Cisco's QoS [du jour] model.)

That said, if you find Cisco auto QoS meets you needs, I see no reason not to use it, again, though, with the understanding that Cisco might change it on you.