Quick Answer
The answer is Frame Relay encapsulation, because it is the only encapsulation type that supports multiple logical subinterfaces on a single serial link by using Data Link Connection Identifiers (DLCIs) to create separate virtual circuits. This allows different VLANs to be mapped to distinct subinterfaces, enabling Layer 2 segmentation across a point-to-point or multipoint topology. On the JNCIA-Junos exam, this concept tests your understanding of serial interface configurations and the limitations of other encapsulations like PPP or HDLC, which cannot support multiple subinterfaces. A common trap is assuming that VLAN tagging alone works on serial links, but Frame Relay is required for subinterface multiplexing. Remember the mnemonic: “Frame for many, PPP for one” — Frame Relay gives you multiple DLCIs, while PPP and HDLC give you only a single logical link.
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. 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.
An engineer is designing a network with two routers connected via a serial link. The link should support multiple logical subinterfaces for different VLANs. Which encapsulation type must be used on the serial interface?
Ethernet
Why wrong: Ethernet is not used on serial interfaces.
PPP
Why wrong: PPP does not support subinterfaces.
Frame Relay
Frame Relay supports subinterfaces for multiple logical circuits.
HDLC
Why wrong: HDLC is Cisco proprietary and does not support subinterfaces.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Frame Relay
Frame Relay is correct because it supports multiple logical subinterfaces (DLCIs) on a single serial link, allowing different VLANs to be mapped to separate virtual circuits. Unlike PPP or HDLC, Frame Relay inherently provides the ability to create point-to-point or multipoint subinterfaces for Layer 2 segmentation.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Ethernet
Why it's wrong here
Ethernet is not used on serial interfaces.
PPP
Why it's wrong here
PPP does not support subinterfaces.
Frame Relay
Why this is correct
Frame Relay supports subinterfaces for multiple logical circuits.
Related concept
Read the scenario before looking for a memorised answer.
HDLC
Why it's wrong here
HDLC is Cisco proprietary and does not support subinterfaces.
Common exam traps
The trap here is that candidates often assume PPP or HDLC can support subinterfaces because they are common WAN encapsulations, but only Frame Relay (and newer technologies like VLAN tagging on Ethernet) provides the logical multiplexing required for multiple subinterfaces on a serial link.
Detailed technical explanation
Frame Relay uses Data Link Connection Identifiers (DLCIs) to create multiple virtual circuits over a single physical serial link, each of which can be assigned to a subinterface. In JUNOS, subinterfaces are configured with `family inet` and a specific DLCI, enabling per-VLAN routing without additional hardware. This is particularly useful in hub-and-spoke topologies where a single serial interface must terminate multiple remote sites with distinct VLANs.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
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FAQ
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: Frame Relay — Frame Relay is correct because it supports multiple logical subinterfaces (DLCIs) on a single serial link, allowing different VLANs to be mapped to separate virtual circuits. Unlike PPP or HDLC, Frame Relay inherently provides the ability to create point-to-point or multipoint subinterfaces for Layer 2 segmentation.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This JNCIA-JUNOS practice question is part of Courseiva's free Juniper Networks 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 JNCIA-JUNOS exam.
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