The answer is DSCP, the Differentiated Services Code Point field in the Layer 3 IP header. This is the correct choice because DSCP, defined in RFC 2474, is designed for end-to-end QoS marking and is preserved across Layer 3 boundaries, unlike Layer 2 fields such as CoS or MPLS EXP which are stripped or rewritten at those boundaries. On the ENCOR 350-401 exam, this question tests your understanding of where QoS markings survive in a multivendor network; a common trap is choosing CoS because it works on switches, but it fails at the router interface. Remember that DSCP rides inside the IP packet itself, so it travels unchanged from source to destination as long as intermediate devices are configured to trust it. A helpful memory tip: DSCP stands for "Doesn't Stop at the Core or Perimeter"—it stays with the packet end-to-end.
350-401 QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
CoS
Why it's wrong here
Incorrect because CoS is a Layer 2 marking and is stripped at Layer 3 boundaries.
✗
IP Precedence
Why it's wrong here
Incorrect because IP Precedence is also a Layer 3 field, but it has only 3 bits, providing fewer classes than DSCP.
✓
DSCP
Why this is correct
Correct because DSCP is a Layer 3 field that is preserved across routers and is supported by most vendors.
Related concept
Read the scenario before looking for a memorised answer.
✗
MPLS EXP
Why it's wrong here
Incorrect because MPLS EXP is used in MPLS networks and may not be supported by all non-Cisco devices.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
DSCP uses 6 bits in the ToS byte of the IPv4 header (or Traffic Class byte in IPv6), allowing 64 possible codepoints (0-63). In a real-world multi-vendor network, DSCP is the standard for DiffServ QoS because it is defined at Layer 3 and is preserved across routers, whereas CoS and MPLS EXP are domain-specific and require mapping at boundaries. A common subtlety is that some non-Cisco devices may strip or re-mark DSCP by default unless explicitly configured to trust it, so the engineer must ensure trust boundaries are set correctly.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
QoS Architecture — This question tests QoS Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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