Do I have to use QOS COSvalues for trunk links between switches?

[darrenriley5]

Hi,

I've been having a debate with a colleague about QOS COS values. My colleague says I need to use COS values across layer 2 trunk links between access layer switches and core switches. My argument is if phones are marking packets with DSCP values I don't need to be concerned with Cos.

The reason I ask is we're implementing a new phone system, the ip phones will  mark RTP traffic wih dscp value EF and Call signaling with DSCP value of CS3. If my understanding is correct I can trust the dscp values of the phones. We are using Cisco 4507 switches which I believe automatically trust dscp values so I would just need a class-map to match the dscp values and apply the output policy map on the egress interfaces as follows?

policy-map -VoIP-Output-Policy

        class VoIP-Bearer-QosGroup

   set dscp ef (Do I need to do this if the packets are already marked with dscp values?)

    priority

   police cir percent 33

        class -Control-QosGroup24

set dscp cs3 (Do I need to do this if the packets are already marked with dscp values?)

bandwidth remaining percent 5

[Joseph W. Doherty]

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Posting

The only time you really should need to use L2 CoS, is when dealing with devices that can't process L3 ToS.

PS:

I don't believe Catalyst switches, such as your 4507, automatically trust QoS tags.

Logically, shouldn't need to set ToS unless it needs changing.  (NB: on some platforms, it's recommended to set anyway, as more efficient than trusting.)

[darrenriley5]

Hi Joseph,

Thanks for the quick response. We are using SUp 6-E Supervisors so maybe this explains it.

"Catalyst 4500 Classic Supervisor switch ports can be configured to statically trust CoS, DSCP, or to

dynamically and conditionally trust Cisco IP phones. By default, with QoS enabled, all ports are set to

an untrusted state.

In contrast, the Catalyst 4500-E Supervisor 6-E does not support trust CoS, as it considers all interfaces

to be trusted (via DSCP-trust) by default; it does, however, support conditional trust."

[Joseph W. Doherty]

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 whatsoever (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

Duh!  Forgot the latest generation of 4500 sups have much enhanced QoS (among other improvements).