Qos questions

[Rajmike675]

My old switch have no mls qos rewrite ip dscp but new cisco C9300 dont have this command how to make sure not to rewrite dscp packages on c9300?

[MHM Cisco World]

trust dscp I think is same as no rewrite the dscp value.

[balaji.bandi]

New IOS XE uses MQC

below document explain the conversation :

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3850-series-switches/118629-technote-qos-00.html

 no mls qos rewrite ip dscp  - this means To disable ToS-to-DSCP rewrite, use the no form of this command.

Cat 9300 as per I know by default its disabled :

The DSCP transparency mode is disabled by default. If it is enabled (mls qos rewrite ip dscp configuration command), DSCP rewrite will not happen.

show run all ( you see the configuration)

check below guide for Cat 9300 (check depends on version) - this is an example :

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-12/configuration_guide/qos/b_1612_qos_9300_cg/configuring_qos.html

[Joseph W. Doherty]

"New IOS XE uses MQC"

@balaji.bandi beat me in responding.

Basically, since about the 3650/3850 series, QoS on Cisco switches is much more like what's been found (for years) on Cisco routers.  Beside this difference, on the older Cisco switches QoS was disabled by default.  The later Cisco switches, I believe, have QoS enabled by default (which, if true, is unlike, I believe, Cisco routers where QoS is still off by default).

Also, with QoS enabled on either Cisco routers or the later Cisco switches, also by default, the packet's ToS values is not changed, unless you configure the device to do so.

[Rajmike675]

I have only auto qos trust in port level is that good enough not to modify any ip dscp package

[MHM Cisco World]

must share config to see it OK or not.

[balaji.bandi]

yes that is correct..

[Joseph W. Doherty]

"I have only auto qos trust in port level is that good enough not to modify any ip dscp package"

Depends on what that particular AutoQoS is doing (it does vary across IOS versions).  It also depends whether any additional manual modifications have been made to the AutoQoS config.

Assuming no manual configuration modifications have been made, I wouldn't expect AutoQoS to override/rewrite ToS, but, again, something you would need to examine for your AutoQoS configuration.

[Rajmike675]

How to check if autoqos overwriting tos and if does how to stop? I dont see any guide on official site

[MHM Cisco World]

good Q, and need deep dive to answer. 
I will check and update you 

[Joseph W. Doherty]

Find whatever policy statements are being used.  Then find what class maps those policy statements are using.  Then check if class maps have "set" statements in them.