When QoS marking is enabled on a Cisco device, which IP header field is modified?

When QoS marking is enabled on a Cisco device, the Type of Service (ToS) field in the IP header is modified. The ToS field is an 8-bit field used to indicate the desired quality of service for the packet. While DSCP (Differentiated Services Code Point) is a common method for QoS marking, it actually uses the first 6 bits of the ToS field. Therefore, modifying DSCP inherently modifies the ToS field. ECN (Explicit Congestion Notification) uses the last 2 bits of the ToS field, but QoS marking primarily refers to the classification and prioritization using the DSCP portion. The Header Checksum is used for error detection and is not modified for QoS marking.