Classification and marking on switches

[sqambera]

Hello,

I am not too experienced with QoS. We have IP phones in our environment connected to access switches which are connected to Nexus on distribution layer. We want to reserve bandwidth for voice on our WAN interfaces of the ASR routers based on the marking of packets by IP phones. I was reading the below link which says:

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91862-cat3750-qos-config.html

QoS is disabled by default on the Catalyst 3750 Switches. While QoS is disabled, all frames/packets are passed-through the switch unaltered. For example, if a frame with CoS 5 and the packet inside the frame with DSCP EF enters the switch, the CoS and DSCP labels are not changed. The traffic leaves with the same CoS and DSCP values as it enters. All the traffic, which includes voice, are delivered on the best effort basis.

Switch#show mls qos QoS is disabled QoS ip packet dscp rewrite is enabled !--- Even though it says QoS ip packet dscp rewrite is enabled, !--- the switch does not alter the

So based on it is it ok if I keep QoS disabled on my access/ distribution layer switches and just configure QoS on router to allocate bandwidth to voice packets based on the dscp marking done by IP phones? I am assuming based on the document link that phone marking will not be altered by the switches as long as QoS is disabled on them.

Thanks for taking time to answer it.

Regards,

Qamber

[chrihussey · Accepted solution]

You are correct in that with it disable the markings will remain intact. However, as stated, all markings are treated the same and all delivered with best effort. So depending on your environment, you may be OK. But, there is also the possibility that at peak traffic times voice quality and reliability could be compromised due to contention with other non delay sensitive data.

[sqambera]

Thanks for the help.

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, that's correct for the 3750s, with the possible issues not enabling QoS might cause, as noted in Chris's post.

However, if you do enable QoS on a 3750, to guarantee VoIP service if there's congestion, 3750's default QoS buffering often requires tuning otherwise you might see drops you don't see with QoS disabled.

[sqambera]

Thanks Joseph. As much as I know same is the case with Nexus switches too, correct?

[Joseph W. Doherty]

Sorry, I have very little experience with the Nexus line.  I don't recall if they are like some of the other new switches that leave QoS marking alone unless you configure them to change it.