Cisco CCNA (200-301)Network FundamentalsHard
A network administrator is configuring Quality of Service (QoS) on a router to prioritize voice traffic over data traffic. Which field in the IPv4 header is typically used for QoS marking?
- AType of Service (ToS)
- BHeader Checksum
- CTime to Live (TTL)
- DProtocol
Show answer & explanationAnswer & explanation
Correct answer: A. Type of Service (ToS)
The Type of Service (ToS) field (now commonly referred to as the Differentiated Services Code Point or DSCP field, which is part of the ToS byte) in the IPv4 header is used by routers to implement Quality of Service (QoS) policies. It allows network devices to classify and prioritize different types of traffic.
Why the other options are wrong
- B. The Header Checksum is used for error detection in the IPv4 header and is not involved in QoS prioritization.
- C. TTL prevents packets from looping indefinitely and is decremented at each hop, not used for QoS marking.
- D. The Protocol field indicates the next-level protocol (e.g., TCP, UDP), not used for QoS marking.
IPv4 Type of Service (ToS) Field
The IPv4 Type of Service (ToS) field (now Differentiated Services Field) is an 8-bit field used to request specific QoS treatment for a packet, enabling prioritization and differentiated services.
- 8-bit field in the IPv4 header.
- Includes the Differentiated Services Code Point (DSCP) bits and Explicit Congestion Notification (ECN) bits.
- Used by routers and switches to classify and prioritize traffic.
Memory trick: To prioritize 'Service', look at the 'Type' in the IPv4 header.