Cisco CCNA (200-301)Network FundamentalsMedium
A network administrator is configuring Quality of Service (QoS) on a Cisco router to prioritize voice traffic over data traffic. Which IPv4 header field is commonly used to mark packets for QoS prioritization?
- AType of Service (ToS)
- BIdentification
- CTime to Live (TTL)
- DProtocol
Show answer & explanationAnswer & explanation
Correct answer: A. Type of Service (ToS)
The Type of Service (ToS) field (now primarily used as the Differentiated Services Code Point or DSCP field) in the IPv4 header is specifically designed for marking packets to indicate desired QoS treatment.
Why the other options are wrong
- B. The Identification field is used for reassembling fragmented IP packets.
- C. TTL prevents packets from looping infinitely, not for QoS marking.
- D. The Protocol field indicates the next-level protocol (e.g., TCP, UDP), not QoS marking.
IPv4 Type of Service (ToS)
A field in the IPv4 header (8 bits) originally intended to specify QoS parameters, now primarily used as the Differentiated Services Code Point (DSCP) field.
- Used to classify packets for different QoS treatments (e.g., priority, delay, throughput).
- The first 6 bits are typically used for DSCP, allowing for 64 different classifications.
- Routers and switches use the DSCP value to make forwarding decisions based on QoS policies.
Memory trick: ToS is for Traffic, TTL for Time, ID for Fragments, Protocol for Next.