- A
The trust boundary can be configured using the 'mls qos trust' command on a switch port to trust the CoS or DSCP value received from an attached device.
Correct. The trust boundary is set with 'mls qos trust' to honor incoming markings.
- B
NBAR (Network-Based Application Recognition) can classify traffic based on application signatures, including HTTP URLs and SSL certificate fields.
Correct. NBAR performs deep packet inspection and can match on many application attributes.
- C
Layer 2 CoS marking uses a 3-bit field in the 802.1Q tag, providing 8 possible values, while DSCP uses 6 bits for 64 values.
Correct. CoS is 3 bits (0–7), DSCP is 6 bits (0–63).
- D
The 'set dscp' command in a policy map can be used to mark packets with a DSCP value, but only on egress interfaces.
Why wrong: Incorrect. The 'set dscp' command can be used on both ingress and egress policy maps, depending on the platform.
- E
MPLS EXP bits are a 3-bit field used for QoS in MPLS networks and are always directly mapped from the IP DSCP value without any configuration.
Why wrong: Incorrect. MPLS EXP bits are 3 bits, but they are not automatically mapped; mapping can be configured using the 'mpls qos' commands.
Quick Answer
The correct answer is that Layer 2 CoS marking uses a 3-bit field in the 802.1Q tag, providing 8 possible values, while DSCP uses 6 bits for 64 values. This is true because classification marking tools in Cisco IOS operate at different OSI layers: CoS (Class of Service) is embedded in the 802.1Q header’s 3-bit Priority Code Point, limiting it to eight values, whereas DSCP (Differentiated Services Code Point) uses six bits in the IP header’s ToS byte, allowing 64 distinct markings for granular traffic treatment. On the ENCOR 350-401 exam, this distinction tests your understanding of trust boundaries and how NBAR performs deep packet inspection to classify traffic before marking; a common trap is confusing the bit counts or assuming CoS applies at Layer 3. Remember that the trust boundary determines where the switch or router honors the incoming marking versus re-marking it. A quick memory tip: “CoS is 3 bits for 8 classes, DSCP is 6 bits for 64—think of the 3 in CoS and the 6 in DSCP as half the bits for a quarter of the values.”
350-401 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 classification and marking tools in Cisco IOS 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
The trust boundary can be configured using the 'mls qos trust' command on a switch port to trust the CoS or DSCP value received from an attached device.
Classification identifies traffic based on various fields, while marking sets the QoS value. Trust boundaries determine where marking is honored. NBAR can perform deep packet inspection for classification. CoS is a Layer 2 marking in 802.1Q frames, while DSCP is Layer 3. MPLS uses EXP bits.
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.
- ✓
The trust boundary can be configured using the 'mls qos trust' command on a switch port to trust the CoS or DSCP value received from an attached device.
- ✓
NBAR (Network-Based Application Recognition) can classify traffic based on application signatures, including HTTP URLs and SSL certificate fields.
Why this is correct
Correct. NBAR performs deep packet inspection and can match on many application attributes.
Related concept
Static NAT maps one inside address to one outside address.
- ✓
Layer 2 CoS marking uses a 3-bit field in the 802.1Q tag, providing 8 possible values, while DSCP uses 6 bits for 64 values.
- ✗
The 'set dscp' command in a policy map can be used to mark packets with a DSCP value, but only on egress interfaces.
Why it's wrong here
Incorrect. The 'set dscp' command can be used on both ingress and egress policy maps, depending on the platform.
- ✗
MPLS EXP bits are a 3-bit field used for QoS in MPLS networks and are always directly mapped from the IP DSCP value without any configuration.
Why it's wrong here
Incorrect. MPLS EXP bits are 3 bits, but they are not automatically mapped; mapping can be configured using the 'mpls qos' commands.
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.
Trap categories for this question
Command / output trap
Incorrect. The 'set dscp' command can be used on both ingress and egress policy maps, depending on the platform.
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: The trust boundary can be configured using the 'mls qos trust' command on a switch port to trust the CoS or DSCP value received from an attached device. — Classification identifies traffic based on various fields, while marking sets the QoS value. Trust boundaries determine where marking is honored. NBAR can perform deep packet inspection for classification. CoS is a Layer 2 marking in 802.1Q frames, while DSCP is Layer 3. MPLS uses EXP bits.
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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