📝 Per hop behavior in DiffServ (12 MCQs)

📖 From Data Communication and Networks • 30. Quality of Service • 12 questions available

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What is Per hop behavior in DiffServ?

Per Hop Behavior defines the forwarding treatment applied to a packet at each DiffServ-compliant router based on its DS codepoint, specifying queuing, scheduling, and dropping policies that collectively produce an aggregate end-to-end service class.

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Hard

📝 All Per hop behavior in DiffServ MCQs

Q1. In an MPLS router, which statement best describes the immediate effect of the per‑hop behavior (PHB) when a labeled packet arrives?

A.The router discards the label and forwards the packet based on its IP header.

B.The router determines the forwarding treatment for the packet solely from the label and applies the associated actions. ✅

C.The router consults a routing table to select the next hop before examining the label.

D.The router uses the label only to update statistics, not to influence forwarding.

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The PHB is defined by the label value and dictates how the router should treat the packet at that hop, such as swapping the label, modifying the EXP bits, or setting QoS parameters. Therefore, the router’s forwarding decision is derived directly from the label, matching option B.

Q2. A packet arrives with a two‑label stack where the outer label maps to LSP A and the inner label maps to LSP B. According to MPLS per‑hop behavior, which label’s PHB is processed first?

A.PHB for LSP B (inner label) is applied before outer label.

B.Both PHBs are applied simultaneously.

C.PHB for LSP A (outer label) is applied first, then the inner label. ✅

D.The router discards the outer label and processes only the inner label.

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: MPLS processing follows a top‑down approach: the outermost label is examined first because it determines the immediate forwarding action, such as label swap or pop. After the outer label is handled, the packet’s new top label (formerly inner) is processed, making option C correct.

Q3. A PHB is configured to decrement the packet’s TTL and then classify the packet for QoS. If a packet arrives with TTL = 1, what will the router do after applying the PHB?

A.The router will drop the packet because the TTL reaches zero. ✅

B.The router will forward the packet after decrementing TTL to zero.

C.The router will forward the packet but ignore QoS classification.

D.The router will increase the TTL before forwarding.

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: TTL is decremented as part of the PHB. When TTL becomes zero, the packet is considered expired and must be discarded to prevent routing loops. The subsequent QoS classification step is never reached, so the router drops the packet, which corresponds to option A.

Q4. In a network where some nodes only support Best‑Effort (BE) PHB, what happens to a packet marked with Expedited Forwarding (EF) when it reaches such a node?

A.The packet is discarded because the node cannot handle EF.

B.The packet retains its EF marking but is forwarded as BE.

C.The node upgrades the packet to a higher‑priority PHB.

D.The packet’s PHB is downgraded to BE, losing its EF priority. ✅

💡 Difficulty: medium | ✅ Correct: D

📖 Explanation: If a node lacks support for EF, it cannot honor the high‑priority treatment. The standard behavior is to downgrade the packet to the highest PHB the node supports, which is BE. Consequently, the packet loses its EF priority, matching option D.

Q5. Which of the following best distinguishes MPLS per‑hop behavior from IPv4 DiffServ per‑hop treatment?

A.MPLS PHB operates on label values, while DiffServ operates on DSCP bits in the IP header. ✅

B.MPLS PHB modifies the IP address, whereas DiffServ only changes TTL.

C.Both MPLS PHB and DiffServ use the same mechanism of queue selection.

D.DiffServ requires label swapping, MPLS does not.

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: MPLS PHB decisions are based on the label attached to the packet, allowing fast label‑based forwarding. In contrast, DiffServ uses the DSCP field within the IP header to select queues. This fundamental difference is captured in option A, making it the correct answer.

Q6. If an MPLS label is associated with two different PHBs—one that swaps the label and another that only pushes a new label—what is the expected behavior?

A.Only one PHB can be active; the router will apply the swap PHB and ignore the push PHB. ✅

B.The router will execute both actions sequentially, first swapping then pushing.

C.The router will generate an error and drop the packet.

D.The router randomly selects one of the PHBs for each packet.

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: A label can be bound to a single forwarding instruction set; having two conflicting PHBs on the same label is undefined. Implementations resolve this by prioritizing one PHB—typically the swap operation—while disregarding the other. Therefore the router applies only the swap PHB, aligning with option A.

Q7. Why can modifying the DSCP field after a label swap in a PHB lead to QoS inconsistency across an MPLS network?

A.Because DSCP values are ignored after a label swap.

B.Because label swaps reset the packet’s QoS classification to default.

C.Because downstream nodes may interpret the altered DSCP differently than intended, breaking end‑to‑end QoS guarantees. ✅

D.Because the DSCP field cannot be changed once a label is present.

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: When a PHB swaps a label, the packet’s forwarding path changes, but the DSCP field may remain unchanged. If the PHB also alters DSCP, downstream routers that rely on the original DSCP for queue selection may misinterpret the packet’s priority, causing a mismatch between intended and actual QoS treatment—hence option C.

Q8. How does static PHB assignment in LDP differ from dynamic PHB assignment in RSVP‑TE?

A.LDP assigns PHBs based on traffic load, RSVP‑TE uses fixed PHBs.

B.LDP does not support PHBs, while RSVP‑TE does.

C.LDP uses pre‑defined PHBs that never change, RSVP‑TE can adjust PHBs per reservation. ✅

D.Both protocols assign PHBs dynamically, but RSVP‑TE does it faster.

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: LDP typically distributes labels with a default PHB (often Best‑Effort) and does not negotiate QoS parameters. RSVP‑TE, on the other hand, can create explicit LSPs with specific PHBs tied to bandwidth reservations, allowing the PHB to be adapted per reservation. This distinction matches option C.

Q9. When designing a PHB for traffic engineering, how should the PHB interact with bandwidth reservation constraints?

A.The PHB should ignore bandwidth reservations to maximize throughput.

B.The PHB must enforce the reserved bandwidth, shaping traffic that exceeds the allocation. ✅

C.The PHB should only prioritize packets without considering bandwidth.

D.The PHB should drop all packets that match the reservation.

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The PHB must respect the amount of bandwidth that has been reserved for an LSP. If traffic exceeds the allocation, the PHB can shape or police the flow to keep it within limits, ensuring that the reservation is honored while still providing appropriate QoS. This aligns with option B.

Q10. Which combination of PHB mechanisms most effectively guarantees both bandwidth reservation and low latency for video streams across multiple MPLS domains?

A.Use RSVP‑TE for explicit bandwidth reservation together with an Expedited Forwarding (EF) PHB in each domain. ✅

B.Apply LDP with Best‑Effort PHB and rely on congestion control.

C.Use static label mapping with a DiffServ PHB only.

D.Deploy MPLS‑TP without any PHB and manage latency at the edges.

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: RSVP‑TE can reserve exact bandwidth for a path, while an EF PHB provides low‑latency treatment by prioritizing packets in the forwarding plane. Combining these mechanisms in each MPLS domain ensures that video streams receive both guaranteed capacity and minimal queuing delay, making option A the most effective solution.

Q11. What is the relationship between the depth of a label stack and the processing overhead imposed by per‑hop behavior?

A.Deeper stacks reduce processing overhead because fewer PHBs are needed.

B.Each additional label adds a fixed amount of overhead, independent of PHB complexity.

C.Processing overhead grows with stack depth because the router must examine and potentially act on each label according to its PHB. ✅

D.Label stack depth has no impact on PHB processing time.

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: Each label in the stack requires the router to perform a lookup, apply the associated PHB, and possibly modify the stack. Consequently, as the number of labels increases, the cumulative processing time and resource usage increase proportionally, which is described by option C.

Q12. In MPLS terminology, what does the abbreviation PHB stand for?

A.Packet Header Block

B.Path Hop Buffer

C.Priority Handling Bit

D.Per‑Hop Behavior ✅

💡 Difficulty: easy | ✅ Correct: D

📖 Explanation: PHB is the acronym for Per‑Hop Behavior, which defines the forwarding and treatment actions applied to a labeled packet at each MPLS hop. This definition directly matches option D.)

🔗 Related Topics (MCQs)

📝 Data flow characteristics in QoS MCQs 📝 QoS definitions in computer networks MCQs 📝 Application sensitivity to QoS parameters MCQs 📝 Flow classes in quality of service MCQs 📝 Flow control techniques for QoS MCQs

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