You are correct.
Just to be clear : Marking and scheduling priority are two different matters. But in your case, they are bound together. See below :
- 2900 / 3500 XL : These switches have two non configurable output scheduling queues. The "low priority" queue for any Cos or port priority from 0 to 3. The "high priority' queue for values from 4 to 7. Note that the "high priority" queue has expedite priority over the "low priority" one. So if your phone marks Cos at 5, you'll get priority over any PC generated traffic.
- 2950 SI : These switches have four configurable queues including one optional expedite priority. You can define which Cos value(s) match a queue using the following commands (note that all queues except expedite one will work in weighted round robin fashion) :
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
# first digit gives queue number, all following digits are cos values falling into that queue number. Therefore, last line says queue N°4 only matches frames ith Cos value equal to 5.
You then need to define the weight for each queue. The value must be set to 0 for the expedite queue matching your voice traffic. All others can be configured with value from 1 to 65536. The more value you provide compared to the addition of all values between queues, the more that queue will be prioritized compared to others except the expedite queue.
Cheers,
Tom
Quick update : Be careful with Cos trust as some drivers allow workstations to modify the CoS value to any value. It is therefore important to have phones being able to reset CoS and DSCP values to 0 when the frame is received on the PC port. Please refer to your phone config guide for more info.
