easyMultiple Choice
350-401 Practice Question: An enterprise is deploying QoS across a network…
An enterprise is deploying QoS across a network that includes both Cisco and non-Cisco devices. The engineer wants to use a marking scheme that is end-to-end and not stripped at Layer 3 boundaries. Which marking field should the engineer use?
⚠ Common exam trap
Cisco often tests the distinction between Layer 2 (CoS) and Layer 3 (DSCP) marking, and the trap here is that candidates confuse 'end-to-end' with 'within a single domain,' leading them to choose CoS or MPLS EXP, which are not preserved across Layer 3 boundaries.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
DSCP
DSCP (Differentiated Services Code Point) is the correct choice because it is defined in RFC 2474 as a Layer 3 marking field in the IP header. Unlike CoS (Layer 2) or MPLS EXP (which is stripped at MPLS boundaries), DSCP markings are preserved across Layer 3 boundaries (routers) and can be used end-to-end across both Cisco and non-Cisco devices, as long as the intermediate devices trust the DSCP value.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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CoS
Why it's wrong here
CoS (Class of Service) is a Layer 2 marking carried in the 802.1Q VLAN header (3 bits), so it exists only within an Ethernet broadcast domain. When a frame passes through a Layer 3 device (router), the VLAN tag is removed and the CoS value is lost, making it ineffective for end-to-end QoS across routed networks. CoS is useful at switch access ports but must be mapped to a Layer 3 field to survive beyond the L2 boundary.
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IP Precedence
Why it's wrong here
IP Precedence is a Layer 3 field, but it uses only the top 3 bits of the IPv4 ToS byte, yielding just 8 possible service classes (0–7). This limits granularity for distinguishing many traffic types; DSCP redefines the same ToS byte using 6 bits, providing 64 codepoints and more flexible QoS classification. Additionally, IP Precedence is now largely deprecated in favor of DSCP, and many modern QoS implementations map DSCP values to Per-Hop Behaviors rather than relying on the older precedence bits.
- ✓
DSCP
Why this is correct
DSCP (Differentiated Services Code Point) is the 6-bit field in the ToS byte of the IP header, making it a true Layer 3 marking that remains intact as packets traverse routers. It is standardized by the IETF and supports up to 64 codepoints, enabling scalable QoS policies like EF (Expedited Forwarding) and AF (Assured Forwarding). Because it is carried in the IP header itself, DSCP is preserved across heterogeneous network devices and is the preferred marking for end-to-end QoS in enterprise and service provider networks.
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MPLS EXP
Why it's wrong here
MPLS EXP is a 3-bit field in the MPLS shim header (Layer 2.5), used to mark priority only within an MPLS domain. It is stripped when the MPLS label is popped at the edge of the MPLS network, and non-Cisco devices may treat it as experimental (hence the name 'EXP') rather than honoring it for QoS. Even within MPLS, EXP values are often copied from IP Precedence or DSCP at the ingress edge, but the field itself is not carried end-to-end across the IP backbone, so it cannot serve as a universal QoS marking.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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