- A
DiffServ uses the 6-bit DSCP field in the IP header to mark packets, allowing up to 64 different classes of service.
Correct. DSCP is 6 bits, providing 64 possible codepoints.
- B
In DiffServ, core routers perform complex classification and marking based on deep packet inspection.
Why wrong: Incorrect. In DiffServ, classification and marking are typically done at the edge; core routers use the DSCP value to apply PHB, reducing complexity.
- C
The Assured Forwarding (AF) PHB group provides four classes, each with three drop precedence levels.
Correct. AF defines four classes (AF1x–AF4x) and three drop precedences (low, medium, high) per class.
- D
DiffServ requires end-to-end signaling using RSVP to reserve bandwidth along the path.
Why wrong: Incorrect. RSVP is used with the IntServ model, not DiffServ. DiffServ does not use signaling.
- E
The Expedited Forwarding (EF) PHB is designed for low-loss, low-latency traffic such as voice.
Correct. EF provides a virtual leased line service with low delay, jitter, and loss, ideal for real-time traffic.
Quick Answer
The correct answer is that the Expedited Forwarding (EF) Per-Hop Behavior (PHB) is designed for low-loss, low-latency traffic such as voice, because the DiffServ model uses the DSCP field in the IP header to classify packets into a limited number of classes, and EF is the specific PHB that provides a virtual dedicated queue for real-time traffic. This is correct because DiffServ relies on per-hop behaviors rather than end-to-end signaling, making it scalable for core networks where routers only need to inspect the six-bit DSCP value to apply the appropriate forwarding treatment. On the ENCOR 350-401 exam, this concept tests your understanding of how DiffServ differs from IntServ, and a common trap is confusing the Assured Forwarding (AF) PHB, which offers multiple drop probabilities, with EF’s strict priority treatment. A key memory tip is to associate EF with “Express Forwarding” for express, low-latency traffic like voice, while AF handles “Assured” but not guaranteed delivery for data classes.
CCNP QoS Practice Question
This 350-401 practice question tests your understanding of qos. 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.
Which three statements about the Differentiated Services (DiffServ) QoS model are true? (Choose three.)
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
DiffServ uses the 6-bit DSCP field in the IP header to mark packets, allowing up to 64 different classes of service.
DiffServ is a class-based model that uses the DSCP field in the IP header to classify traffic. It provides per-hop behavior (PHB) and is scalable because core routers only need to inspect the DSCP field. The model does not guarantee end-to-end bandwidth reservation like IntServ does; instead, it relies on traffic conditioning at the edge.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
DiffServ uses the 6-bit DSCP field in the IP header to mark packets, allowing up to 64 different classes of service.
- ✗
In DiffServ, core routers perform complex classification and marking based on deep packet inspection.
Why it's wrong here
Incorrect. In DiffServ, classification and marking are typically done at the edge; core routers use the DSCP value to apply PHB, reducing complexity.
- ✓
The Assured Forwarding (AF) PHB group provides four classes, each with three drop precedence levels.
Why this is correct
Correct. AF defines four classes (AF1x–AF4x) and three drop precedences (low, medium, high) per class.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
DiffServ requires end-to-end signaling using RSVP to reserve bandwidth along the path.
Why it's wrong here
Incorrect. RSVP is used with the IntServ model, not DiffServ. DiffServ does not use signaling.
- ✓
The Expedited Forwarding (EF) PHB is designed for low-loss, low-latency traffic such as voice.
Why this is correct
Correct. EF provides a virtual leased line service with low delay, jitter, and loss, ideal for real-time traffic.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 350-401 question test?
QoS — This question tests QoS — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: DiffServ uses the 6-bit DSCP field in the IP header to mark packets, allowing up to 64 different classes of service. — DiffServ is a class-based model that uses the DSCP field in the IP header to classify traffic. It provides per-hop behavior (PHB) and is scalable because core routers only need to inspect the DSCP field. The model does not guarantee end-to-end bandwidth reservation like IntServ does; instead, it relies on traffic conditioning at the edge.
What should I do if I get this 350-401 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.
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