Which option is a Layer 2 QoS marking?
- DSCP
- QoS group
- EXP
- CoS
Explanation: CoS (Class of Service) is the standard Layer 2 Quality of Service marking used with Ethernet frames. It consists of a 3-bit field located within the Layer 2 header, which provides eight different priority levels for traffic as it moves across a switched network. While CoS is used for wired Ethernet connections, a similar 3-bit field called TID (Traffic Identifier) is used for Layer 2 marking in wireless 802.11 MAC frames.
In contrast, other options like DSCP (Differentiated Services Code Point) are Layer 3 markings. DSCP utilizes a 6-bit value within the IPv4 Type of Service (ToS) field or the IPv6 Traffic Class field, providing more granular classification that carries end-to-end significance across routers. Because Layer 2 headers are often stripped and rewritten by routers, CoS markings are typically used to maintain priority only within a local broadcast domain or across trunk links before being mapped to Layer 3 markings for wider network transit.
Related exam: Modules 1 – 7: Checkpoint Exam: Network Switching Answers (CCNP ENCOR v9)
