hardMultiple Select
350-401 Classification Practice Question
Which three statements about classification and marking in a QoS architecture are true? (Choose three.)
⚠ Common exam trap
The trap here is assuming MPLS EXP is immutable in the core or that marking must always happen at the source — candidates confuse design best practices with protocol capabilities.
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
✓
Classification can be based on the source IP address, destination port, or protocol type using an access control list.
Option A is correct because classification commonly uses ACLs to match fields such as source IP address, destination port, and protocol type, which are exactly the Layer 3/Layer 4 criteria listed. Option B is correct because Layer 3 marking uses the 6-bit DSCP field in the IP header, giving 2^6 = 64 values, whereas the older IP Precedence field is only 3 bits, giving 2^3 = 8 values. Option E is correct because NBAR performs deep packet inspection and can recognize applications by examining payload content up to Layer 7, beyond simple header-based classification. Option C is not correct because the MPLS EXP field can be modified within the MPLS core, for example at label swap or PHP operations, and is not restricted to ingress-only marking. Option D is not correct as a universal QoS statement because marking is often recommended near the source or at the trust boundary, but it is not an absolute rule and can be performed at aggregation or edge devices depending on policy and trust models.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Classification can be based on the source IP address, destination port, or protocol type using an access control list.
Why this is correct
Classification examines packet header fields, and an ACL matches source IP address, destination port, or protocol type, so traffic can be sorted into classes before marking. This satisfies the stem's requirement for a header-based classification mechanism.
- ✓
Marking at Layer 3 uses the DSCP field, which provides 64 possible values, while IP Precedence provides only 8.
Why this is correct
DSCP occupies six bits of the Layer 3 IP header's ToS byte, yielding 64 distinct values, whereas the legacy IP Precedence field uses only three bits, giving 8. This satisfies the stem's requirement for accurate Layer 3 marking detail, confirming the statement as true.
- ✗
The MPLS EXP field is used to mark packets only at the ingress of an MPLS network and is never changed within the core.
Why it's wrong here
Incorrect: MPLS EXP can be changed within the MPLS core by LSRs for QoS policies.
- ✗
Marking should be performed as close to the source as possible to ensure consistent treatment across the network.
Why it's wrong here
Incorrect: Marking should be done at the trust boundary, not necessarily as close to the source; source devices may be untrusted.
- ✓
NBAR (Network Based Application Recognition) can classify traffic by inspecting the payload up to Layer 7.
Why this is correct
NBAR inspects packet payloads deep into the application layer, identifying protocols and applications by signature rather than relying solely on Layer 3 or 4 headers. This deep packet inspection capability is what enables granular classification beyond port-based methods.
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
Go deeper
Related to this question
About these practice questions
This 350-401 question is part of Courseiva's 1,923-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.