- A
The VC is configured as an OSPF area border router and is not creating a summary for the failing VLANs.
Why wrong: In a single-area OSPF, there is no ABR role.
- B
The VC's MAC address table is not learning the upstream router's MAC address on the correct VLAN.
Why wrong: MAC address table is for L2 switching; the issue is routing.
- C
The IRB interface for the failing VLANs does not have OSPF enabled, so the upstream router does not have a route to those subnets, and return traffic is dropped.
For the upstream router to send return traffic, it must have a route to the VLAN subnet; if the VC does not advertise the subnet via OSPF, the router may use a default route but might send traffic to a different next-hop.
- D
The default route from OSPF is not being installed because there is a static default route with lower preference.
Why wrong: If a static default exists with lower preference (e.g., 5), it would be preferred, but then all VLANs would have internet access via that route. The symptom is per-VLAN failure.
OSPF on IRB Interfaces: VLAN Internet Access
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
You are a network engineer at a company that uses a pair of Juniper EX4300 switches in a virtual chassis (VC) configuration for the campus core. The VC is running OSPF with a single area and has multiple uplinks to an upstream router. The router is advertising a default route via OSPF. You want all inter-vlan traffic within the VC to be switched, but internet-bound traffic should be routed via the default route. You notice that some VLANs are not able to reach the internet while others can. All VLANs have a default gateway on the VC, which is the same IP (the VC's management IP). Users in the failing VLANs can ping the default gateway but cannot ping the upstream router's interface IP. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Quick Answer
The answer is that the IRB interface for the failing VLANs does not have OSPF enabled, preventing the upstream router from learning a route back to those subnets. In a Juniper virtual chassis running OSPF on IRB for VLAN internet access, each IRB interface must be explicitly configured under the OSPF protocol so the switch advertises the connected subnet. Without this, the upstream router lacks a return route for traffic sourced from those VLANs, causing asymmetric routing where outbound packets reach the internet but return traffic is dropped. This question tests your understanding of OSPF route advertisement and IRB integration, a common trap on the JNCIA-Junos exam where candidates assume a default route learned via OSPF applies globally to all VLANs. Remember that OSPF does not automatically include every IRB interface; you must enable it per interface. A useful memory tip: “No OSPF on the IRB means no route back—ping the gateway works, but the internet is black.”
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 IRB interface for the failing VLANs does not have OSPF enabled, so the upstream router does not have a route to those subnets, and return traffic is dropped.
Option C is correct because OSPF must be enabled on the IRB interface of each VLAN for the VC to advertise those subnets to the upstream router. If a VLAN's IRB interface does not have OSPF enabled, the upstream router does not learn a route to that subnet, and when the VC sends traffic to the upstream router, the router has no return route and drops the packets. This explains why some VLANs can reach the internet (those with OSPF on their IRB) and others cannot. Option A is incorrect because the VC is not an area border router in a single-area OSPF design; it is just an internal router. Option B is incorrect because the MAC address learning issue would affect all VLANs, not just some. Option D is incorrect because the default route from OSPF is global and not per-VLAN; the issue is not about preference but about missing routes for the specific subnets.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 VC is configured as an OSPF area border router and is not creating a summary for the failing VLANs.
Why it's wrong here
In a single-area OSPF, there is no ABR role.
- ✗
The VC's MAC address table is not learning the upstream router's MAC address on the correct VLAN.
Why it's wrong here
MAC address table is for L2 switching; the issue is routing.
- ✓
The IRB interface for the failing VLANs does not have OSPF enabled, so the upstream router does not have a route to those subnets, and return traffic is dropped.
Why this is correct
For the upstream router to send return traffic, it must have a route to the VLAN subnet; if the VC does not advertise the subnet via OSPF, the router may use a default route but might send traffic to a different next-hop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The default route from OSPF is not being installed because there is a static default route with lower preference.
Why it's wrong here
If a static default exists with lower preference (e.g., 5), it would be preferred, but then all VLANs would have internet access via that route. The symptom is per-VLAN failure.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Routing Fundamentals — This question tests Routing Fundamentals — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The IRB interface for the failing VLANs does not have OSPF enabled, so the upstream router does not have a route to those subnets, and return traffic is dropped. — Option C is correct because OSPF must be enabled on the IRB interface of each VLAN for the VC to advertise those subnets to the upstream router. If a VLAN's IRB interface does not have OSPF enabled, the upstream router does not learn a route to that subnet, and when the VC sends traffic to the upstream router, the router has no return route and drops the packets. This explains why some VLANs can reach the internet (those with OSPF on their IRB) and others cannot. Option A is incorrect because the VC is not an area border router in a single-area OSPF design; it is just an internal router. Option B is incorrect because the MAC address learning issue would affect all VLANs, not just some. Option D is incorrect because the default route from OSPF is global and not per-VLAN; the issue is not about preference but about missing routes for the specific subnets.
What should I do if I get this JNCIA-JUNOS question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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