You have a policy that must differentiate between 80 distinct traffic classes on a router. Which feature do you use for marking?

CoS (Class of Service) is encoded in 3 bits of the 802.1Q VLAN tag, providing only 8 distinct values (0-7) - far below the 80 classes required.

DSCP uses 6 bits in the IP Differentiated Services field, providing 64 possible values (0-63) - still insufficient to differentiate 80 distinct classes.

QoS groups are a Cisco IOS internal marking mechanism not constrained by the bit width of any standard protocol header field - they can be assigned values sufficient to distinguish 80 or more distinct traffic classes depending on the platform. Unlike DSCP (limited to 64 values by its 6-bit field) or CoS and IP Precedence (limited to 8 values each by their 3-bit fields), QoS groups are applied locally on the router and used in policy-map class matching, making them the only listed option that can represent 80 distinct classes. Traffic is classified into a QoS group and then acted upon by subsequent policy stages.

IP Precedence uses 3 bits in the IP Type of Service byte, yielding only 8 distinct values (0-7) - far fewer than the 80 classes required.