QoS - Marking with Switch QoS not enabled

[sy_price]

Hi, 

Marking traffic from source. E.g. VTC with DSCP Value. 

VTC connects into Switch. Switch has not been enabled for QoS (qos mls has not been enabled).

Will the switch just pass through the traffic with DSCP value or with the switch overwrite the traffic with the switches internal dscp value (0)?

Thanks

Simon

[Mark Malone]

You should still honour it at the port and uplink , mls qos trust dscp , it can depend on the hardware and whether it will trust DSCP markings by default oir rewrite , ios-xe and nx-os will ios wont on some devices , you could tell by spanning local port and see if the marking is altered in wireshark

[nixpengu1n]

Hello,

QoS mechanisms hugely depends on a switch model you use. Please provide it at a first place.

In general you have two options: either trust an incoming COS / DSCP values on ports and carry them over to uplink ports, or do not trust any COS / DSCP values and classify traffic on ingress queue to assign new COS / DSCP value and police / shape / prioritize traffic accordingly. 

[Joseph W. Doherty · Accepted solution]

As the other posters noted, what may happen depends on the device.

Generally, older switches, by default, reset QoS settings if not enabled to trust what they receive. However, this is also only if QoS is enabled by default, and on such switches it generally isn't. I.e. on the older switches, the default generally is that QoS isn't enabled, and if not enabled, markings will not be changed. if the switch's QoS is enabled, the default then usually becomes QoS markings are reset to zero.

Recent switches behave much like routers, that by default, don't reset QoS markings unless configured to do so. I.e. they implicitly trust a frame's/packet's QoS markings. However, the device, also by default, may not offer any special treatment, for specific QoS markings, unless configured to do so.

[sy_price]

Thanks for your time and all the answers given.

Simon

[Mark Malone]

For 3750 you should apply it to access and uplink they dont honor by default , it shouldnt alter the the DSCP marking though if it was cos it could rewrite it but been a long time since i confirmed but all my 37s where the source marks like cisco phones we trust oin access and uplinks