CCNA: Cisco Certified Network Associate — Question 660

Which IP header field is changed by a Cisco device when QoS marking is enabled?

Answer options

• A. ECN
• B. Header Checksum
• C. Type of Service
• D. DSCP

Correct answer: D

Explanation

The correct answer is D, DSCP, as this field is specifically designed for QoS marking and is altered to prioritize traffic. The ECN field (A) is used for congestion notification and is not directly related to QoS. The Header Checksum (B) is a value calculated for error-checking the header and doesn't change with QoS. The Type of Service (C) field is an older standard that has largely been replaced by DSCP for traffic classification.