Hopefully this is a straightforward question as I am new to Qos.
My question is if I have a network of switches and I enable QoS on only one switch (using 'mls qos') and mark traffic from Cisco IP phones with a default CoS of 5, when the marked packets travel across a trusted trunk link to switches that do not have QoS enabled globally (i.e. do not have 'mls qos'), do the switches ignore the CoS values and use 'best effort' delivery or do they just trust the CoS values by default and forward those packets before any others?
Switches that do not have QoS enabled do not alter the QoS markings. They forward them on as they are, so in your case if one switch marks a packet with CoS as it traverses other switches and gets to a router the CoS will be 5.
The behaviour, to some extent, depends on the switch. For example, on a Cisco 2950, even if you have not enabled 'mls qos', the default behaviour is to *not* trust any CoS/DSCP markings, so all your markings will be lost.
You need to be wary of the type of switches that you are dealing with and their default behaviour.
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