The answer is that set dscp default explicitly marks the DSCP field to a value of 0, which corresponds to the default best-effort per-hop behavior (PHB) as defined in RFC 2474. This command is used within the class-default to ensure that any traffic not matching the higher-priority user-defined classes, such as CRITICAL_DATA or BULK_DATA, is intentionally assigned the lowest possible priority, overriding any implicit or inherited markings. On the ENCOR 350-401 exam, this concept tests your understanding of QoS marking policies and the distinction between leaving a packet unmarked versus explicitly setting it to DSCP 0; a common trap is confusing "default" with "no marking" or assuming it inherits the interface trust setting. The key insight is that "set dscp default" is an explicit action that forces best-effort treatment, making it a deliberate policy choice rather than a passive fallback. A helpful memory tip: think of "default" as "zero-effort" — DSCP 0 means no special forwarding treatment.
350-401 QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Given the following configuration:
policy-map MARKING_POLICY
class CRITICAL_DATA
set dscp af31
class BULK_DATA
set dscp af11
class class-default
set dscp default
What is the effect of the set dscp default command in the class-default?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It sets the DSCP value to 0, which is the default best-effort marking.
The 'set dscp default' command explicitly sets the DSCP field to a value of 0, which corresponds to the default best-effort per-hop behavior (PHB) as defined in RFC 2474. This ensures that any traffic not matching the user-defined classes (CRITICAL_DATA or BULK_DATA) is marked with the lowest priority, which is the standard behavior for class-default in a marking policy.
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.
✓
It sets the DSCP value to 0, which is the default best-effort marking.
It is invalid because class-default cannot have a set action.
Why it's wrong here
Class-default can have set actions; it is valid.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that 'default' means 'leave the original value unchanged' or that it refers to a specific high-priority default like voice, rather than the actual DSCP value of 0 for best-effort traffic.
Trap categories for this question
Command / output trap
The command explicitly sets it to 0, not preserving the original.
Detailed technical explanation
How to think about this question
The 'default' keyword in the 'set dscp' command is a shorthand for DSCP 0, which is the same as the IP precedence value of 0 (routine). In a QoS policy, marking all class-default traffic with DSCP 0 ensures that any unclassified traffic receives the lowest possible priority in downstream devices, preventing it from interfering with higher-priority flows. This is particularly important in a DiffServ domain where consistent marking across all traffic is required.
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: It sets the DSCP value to 0, which is the default best-effort marking. — The 'set dscp default' command explicitly sets the DSCP field to a value of 0, which corresponds to the default best-effort per-hop behavior (PHB) as defined in RFC 2474. This ensures that any traffic not matching the user-defined classes (CRITICAL_DATA or BULK_DATA) is marked with the lowest priority, which is the standard behavior for class-default in a marking policy.
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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