The answer is that OSPF router IDs must be unique across all routers in the OSPF domain. This uniqueness is critical because OSPF uses the router ID as the primary identifier for each router when forming neighbor adjacencies and exchanging link-state advertisements. If two routers share the same router ID, the OSPF process cannot distinguish between them, causing adjacency failures and potentially creating routing loops, as specified in RFC 2328. On the JNCIA-Junos exam, this concept often appears in questions about OSPF configuration and neighbor state troubleshooting, with a common trap being that a router ID only needs to be unique within an area—it must actually be unique domain-wide. A helpful memory tip: think of the router ID as a social security number for OSPF—no two routers can share the same one, or the network cannot tell them apart.
JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking 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.
A network administrator is configuring a new OSPF network. Which statement about OSPF router IDs is correct?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Router IDs must be unique across all routers in the OSPF domain.
In OSPF, the router ID must be unique across the entire OSPF domain to ensure proper neighbor adjacency formation and loop-free routing. If two routers share the same router ID, OSPF cannot distinguish between them, leading to adjacency failures and potential routing loops. This uniqueness requirement is specified in RFC 2328.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
If no router ID is configured, the system uses the lowest IP address on an active interface.
Why it's wrong here
It uses the highest IP address.
✗
The router ID must be an IP address from a directly connected interface.
Why it's wrong here
Router ID can be any unique IP address, not necessarily directly connected.
✗
The router ID is automatically derived from the MAC address.
Why it's wrong here
It is derived from the highest loopback or physical IP.
✓
Router IDs must be unique across all routers in the OSPF domain.
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the OSPFrouter ID selection process with Cisco's behavior (which uses the highest loopback IP, then highest physical IP), but Junos uses the highest IP address on any active interface, with loopback interfaces taking precedence over physical interfaces.
Detailed technical explanation
How to think about this question
Under the hood, the OSPFrouter ID is a 32-bit number used to identify the originating router in LSAs and to elect the Designated Router (DR) and Backup Designated Router (BDR). In Junos, you can explicitly set the router ID using the 'set routing-options router-id' command, which overrides any automatic selection. A common real-world scenario is when a router has multiple loopback interfaces — Junos will use the highest IP address among them as the router ID if none is manually configured.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Router IDs must be unique across all routers in the OSPF domain. — In OSPF, the router ID must be unique across the entire OSPF domain to ensure proper neighbor adjacency formation and loop-free routing. If two routers share the same router ID, OSPF cannot distinguish between them, leading to adjacency failures and potential routing loops. This uniqueness requirement is specified in RFC 2328.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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