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Voice QoS, What's the difference between trusting COS vs DSCP? posted 01/24/2007
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I'm trying to understand the difference between a switch trusting a COS
value vs trusting a DSCP value.

With my current understanding, a phone would generate a packet (L3) and mark
this packet with an appropriate DSCP value. This packet would then be
encapsulated in a L2 frame as it leaves the phone. The DSCP value would be
copied to the the L2 frame COS value to closest to it. (i.e. 46 DSCP would
become 5 COS by default). This L2 frame would enter the wire and go to the
connected switch.

At this point, the switch has the option to either trust the COS value (on
the L2 frame) or the encapsulated DSCP value in the L3 packet.

I want to understand the ramifications of choosing one over the other.

Based on my current understanding, I would want to trust DSCP as the switch
and all devices along the path would keep the L3 DSCP value intact, and
strip and reapply the COS value whenever L3 to L2 back to L3 encapsulation
occurs. Where by trusting the L2 COS value, as soon as the packet reaches a
L3 interface, the frame is de-encapsulated and the COS is tossed. Since we
are trusting COS, and no COS exists, essentially all QoS is lost at this

For example (best viewed with fix width font):

DSCP = 46

L3  IP address     SVI 5                    SVI 5    IP address
L2    PHONE <-----> SW1 <------> SW2 <-----> SW3 <-----> SBC
VLAN    5            5            5           5

In this above case, trusting DSCP, the packet would maintain it's QoS value
through the entire network as it passes from L2 to L3 and back. But if trust
COS, SW1 would strip the L2 frame removing the COS value, and essentially
losing any further QoS as we no longer have a value to trust.

Is this correct? Or have I missed something in the behavior?

What's the difference between trusting COS vs DSCP?