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Priority/DSCP Question

I'm fairly new to the QoS world and have done a couple configurations, but I've mostly been working off of templates and would really like to understand a few things. Most documents that discuss QoS configurations show that DSCP values are the primary driver in determining Class of Service, which in turn determines which Queue the traffic is ultimately put in.

One of my questions is if I have a phone that markes it's traffic with Priority 5 off the get go, do I need to even mess with DSCP values? I'm under the impression that CoS values are synonomous with Priority (aka Precedence) values. If all this is true, and the traffic is already marked with a CoS value, and hence is already assigned to a Queue, shouldn't I be good-to-go as long as I have my Class, Policy, and Service maps set up?

Here's what I believe a sample QoS config would look like with Polycom phones that already mark their traffic with a priority of 5.

class-map match-any VoIP

match precedence 5

policy-map QoS

class VoIP

priority percent 35

class class-default


interface Serial0/0/0

ip address x.x.x.x x.x.x.x

service-policy output QoS

I'm pretty sure I have my theories and understandings messing up somewhere. I just need it to click and would appreciate any help I could get.


Super Bronze

Priority/DSCP Question


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When you describe a phone marking priority 5, you need to note whether you're describing L2 CoS 5 or L3 IP Precedence 5.  (For example, you policy is testing for IP Prec 5.)

DSCP is the current RFC for using the IP header's ToS byte for QoS purposes.

If a phone is marking either (or both) L2 CoS 5 and L3 IP Prec 5, would recommend you remark packets to DSCP EF (ASAP).

Technically, you can often do the same QoS processing with any marking, so you don't have to remark, although CoS markings, by default, are usually lost at the 1st L3 hop.  (NB: on Cisco switches, enabling QoS, by default, often resets CoS and/or ToS on first such switch port ingress.)

Whether your device even treats traffic differently often requires a non-default configuration.

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