[cisco-voip] How to check for QoS?

Tue Jan 6 10:28:37 EST 2009

http://www.medicineforthedefense.com/
Thanks so much for everyone's input, it's very much appreciated!

"show policy-map" on both our voice and data routers is returning with
nothing, so I'm taking that to mean we don't have any policies applied
on those.

I was able to get the packet capture into Wireshark, and the RTP packets
are showing the following:

Differentiated Services Field: 0xb8 (DSCP 0x2e: Expedited Forwarding;
ECN: 0x00)

                1011 10.. = Differentiated Services Codepoint: Expedited
Forward (0x2e)

Based on my very limited knowledge, that does look like we're applying
QoS somewhere? If that is the case, would it be applied at the
switchports?

From: Sean Walberg [mailto:swalberg at gmail.com] 
Sent: Monday, January 05, 2009 5:37 PM
To: James Buchanan
Cc: Weigand, John V.; cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] How to check for QoS?

If you get the packet capture into Wireshark, you can (in addition to
checking for the DSCP == EF) measure the latency to see if QoS is doing
its job.

>From a router, though, you can check your policy maps with "show
policy-map" or "show policy-map interface", it will tell you if a policy
is applied and what it's doing.  You're looking for a priority queue.

The Cisco QoS SRND is a good way to understand QoS.  It's hefty, but
most of the details are duplicated for every model of switch.

Sean

On Mon, Jan 5, 2009 at 4:07 PM, James Buchanan <jbuchanan at ctiusa.com>
wrote:

Hi John,

The biggest thing you are looking for is that the voice traffic is being
marked and that the marking is being recognized throughout the network.
A packet capture is the quickest way to determine this. In the packet
capture, you will see a field for Differentiated Services. For traffic
that is the actual audio payload, you should see the Differentiated
Services field as Expedited Forwarding. You will want to check packets
going in each direction. 

On the switches, for ports that have a phone plugged in you should see
that the switchport is configured to trust cos and is configured to
trust based on the device being a cisco phone (mls qos trust cos and mls
qos trust cisco-phone). What these commands look like can vary according
to the model of switch. 

For any connection that is a voice server or voice gateway, the
switchport should trust the dscp value (mls qos trust dscp). This should
also be true on uplinks from switch to switch and from switch to WAN
router.

On the WAN router, depending upon the speed of your connection you
should be using some sort of low latency queuing and/or traffic shaping.

Thanks,

James

From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Weigand, John
V.
Sent: Monday, January 05, 2009 4:02 PM
To: cisco-voip at puck.nether.net
Subject: [cisco-voip] How to check for QoS?

	Litigation Management

Serious Medicine for the Defense (r)
<http://www.medicineforthedefense.com/>  

	I've been tasked with checking to see if our VAR ever setup any
QoS at all, and if so, what type and how it's set. Unfortunately,
although I've become fairly proficient at CallManager itself, I'm pretty
new to the inner workings of everything at the actual network level.

I know there's a few different ways to set it all up, and I've tried
poking around, but I'm not really even sure where to begin looking. Does
anyone have any pointers as to how I might be able to track down some of
this info? Is there anything I can tell from a packet capture of a call
from between two of our sites? I do also have read only access to the
switches/routers on the network. 

Thanks in advance!

	John V. Weigand
Help Desk Support/Executive Support

Litigation Management, Inc.
300 Allen-Bradley Drive
Suite 200
Mayfield Heights, OH 44124

Tel: 440-484-2000
Fax: 440-484-2009
Cell: 
email: jvw at medicineforthedefense.com 

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Help Desk Support/Executive Support

Litigation Management, Inc.
300 Allen-Bradley Drive
Suite 200
Mayfield Heights, OH 44124

Tel: 440-484-2000
Fax: 440-484-2009
Cell: 
email: mailto:jvw at medicineforthedefense.com 

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-- 
Sean Walberg <sean at ertw.com>    http://ertw.com/

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