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Commonly confused · Collaboration, Voice & AV

DSCP vs CoS: What's the Difference?

DSCP is a 6-bit QoS marking in the IP header that survives across routed networks end to end. CoS is a 3-bit priority field in the 802.1Q VLAN tag that exists only on trunk links at Layer 2 and is lost when a frame is routed or sent untagged.

vs

CoS (802.1p)

DSCPCoS
OSI layerLayer 3 (IP header)Layer 2 (802.1Q tag)
Field size6 bits (64 values)3 bits (8 values, 0-7)
LocationDS field, formerly ToS bytePCP field in the VLAN tag
Survives routingYesNo; removed with the Layer 2 header
Voice markingEF (46)5
Video/call signalingAF41 (34) for video, CS3 (24) for signaling4 for video, 3 for signaling
Remember it

CoS lives in the frame and dies at the router; DSCP lives in the packet and travels end to end.

Exam tip

Know EF (DSCP 46) for voice and CoS 5 for voice, and that CoS requires an 802.1Q tag. Trust boundary questions ask where markings from endpoints like IP phones should be trusted.

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