General discussion


Catalyst qos

By summers ·
Hi all,

I am a junior network engineer.
I am currently working on catalyst 3650 and wondering if my understanding is correct about qos.

1) a dscp or cos value in incoming packet gets translated to internal dscp value.

2)internal dscp value gets translated to cos value.

3) packets get assinged to a certain queue based on cos value.(by looking at cos-queue map)

I have a couple of questions here.

Q1) why internal dscp value gets translated to cos value to assign to a queue. It seems like a waste.

Q2) When a port is configured as an access port or
trunk port, can I put "qos trust dscp"?
I mean, can non routed port looks at dscp value?

Thanks in advance,


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by CG IT In reply to Catalyst qos

I'll assume that you know the ins and outs of Quality of Service on Cisco routers/switches.

Once QoS is enabled all packets must be assigned a value for QoS. at some point in packet travel QoS is encountered, therefore QoS must have values to process the packet.

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by mshavrov In reply to Catalyst qos

DSCP is Layer 3 QoS packet marking, and is based on the ToS (Type-of-Service) field in your IP headser.

CoS is is Layer 2 frame prioritization, based on the ISL of 802.1Q header.

So, your switch should have some way to convert values between DSCP and CoS to handle QoS properly. So, if port is L2 trunk, it uses Cos, if it's L3, it uses DSCP (ToS).

When you configure port with "qos trust dscp", you trust your PC, connected behind the phone, to set QoS parameters for data traffic.

Good luck,


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by summers In reply to Catalyst qos

Thanks for all the answers!!
I appreciate your help!

I would like to use Cat3560 as a access switch.
If I configure the port like below, does the switch look at the dscp(L3) value of incoming packet and put voice packets into priority queue? or I have to use "mls qos trust cos"?

interface FastEthernet0/1
switchport trunk encapsulation dot1q
switchport mode trunk
switchport voice vlan 2
mls qos trust dscp
priority-queue out
spanning-tree portfast



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