Which statement about DSCP and CoS in QoS is accurate?

Which statement about DSCP and CoS in QoS is accurate?

• A. DSCP marks IP header priority; CoS marks Ethernet frame priority.
• B. DSCP and CoS are interchangeable.
• C. DSCP marks length of packet.
• D. CoS marks header of IP.

Answer: (A)

In QoS, labeling happens at different layers to guide how traffic is handled. DSCP marks the IP header to indicate the packet’s desired level of service as it moves through IP routers, using the Differentiated Services field in IPv4 (and the Traffic Class in IPv6). CoS marks Ethernet frames, carried in the 802.1Q VLAN tag (the PCP field), guiding LAN switches on how to queue or prioritize frames within the local network. These mechanisms aren’t interchangeable; they operate at different layers, though devices can map DSCP to CoS values to maintain priority across networks when policies exist. So the statement that DSCP marks IP header priority and CoS marks Ethernet frame priority correctly describes their roles. The other options misstate one or both functions—for example, DSCP does not mark the packet length, and CoS does not mark the IP header.

In QoS, labeling happens at different layers to guide how traffic is handled. DSCP marks the IP header to indicate the packet’s desired level of service as it moves through IP routers, using the Differentiated Services field in IPv4 (and the Traffic Class in IPv6). CoS marks Ethernet frames, carried in the 802.1Q VLAN tag (the PCP field), guiding LAN switches on how to queue or prioritize frames within the local network. These mechanisms aren’t interchangeable; they operate at different layers, though devices can map DSCP to CoS values to maintain priority across networks when policies exist. So the statement that DSCP marks IP header priority and CoS marks Ethernet frame priority correctly describes their roles. The other options misstate one or both functions—for example, DSCP does not mark the packet length, and CoS does not mark the IP header.