- A
The OSPF router-id is automatically derived from the MAC address of the first Ethernet interface.
Why wrong: The router-id is not derived from the MAC address.
- B
If the router-id is changed using the 'router-id' command, the change takes effect immediately without any additional action.
Why wrong: Changing the router-id requires a reload or a 'clear ip ospf process' command to take effect.
- C
The router-id must be the same on all routers in a single OSPF area.
Why wrong: The router-id must be unique within the OSPF domain.
- D
When no 'router-id' is configured, a loopback interface with the highest IP address is preferred over a physical interface for the router-id.
Loopback interfaces are preferred due to their stability.
- E
The 'show ip ospf' command displays the current OSPF router-id.
This command shows the router-id among other OSPF details.
Quick Answer
The correct answers are D and E, because OSPFv2 selects its router-ID by preferring any explicitly configured router-id, then the highest IP address on any loopback interface, and finally the highest IP on a physical interface, while the 'show ip ospf' command reliably displays the current router-ID in use. This selection process ensures a stable, unique identifier for each router, as loopback interfaces rarely go down, preventing unnecessary OSPF adjacencies from resetting. On the CCNA 200-301 v2 exam, this topic tests your understanding of OSPF router-ID selection and verification in a single-area configuration, often appearing in multiple-choice questions that include common traps like confusing MAC addresses with IP addresses or assuming a router-ID change takes effect immediately without a reload or OSPF process reset. A key memory tip is "Loopbacks first, highest wins" — remember that loopback addresses always beat physical interface addresses, and if you change the router-ID, you must clear the OSPF process or reload for it to take effect.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. 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.
Which TWO statements correctly describe OSPFv2 router-id selection and verification in a single-area configuration?
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
When no 'router-id' is configured, a loopback interface with the highest IP address is preferred over a physical interface for the router-id.
OSPFv2 selects the router-ID based on the highest IP address of any loopback interface when no explicit 'router-id' is configured, making D correct. The 'show ip ospf' command displays the current router-ID, verifying choice E. Option A is incorrect because the router-ID is derived from IP addresses, not MAC addresses. Option B fails because changing the router-ID requires a reload or clearing the OSPF process to take effect. Option C is wrong because each router must have a unique router-ID; they do not need to match across the area.
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.
- ✗
The OSPF router-id is automatically derived from the MAC address of the first Ethernet interface.
Why it's wrong here
The router-id is not derived from the MAC address.
- ✗
If the router-id is changed using the 'router-id' command, the change takes effect immediately without any additional action.
Why it's wrong here
Changing the router-id requires a reload or a 'clear ip ospf process' command to take effect.
- ✗
The router-id must be the same on all routers in a single OSPF area.
Why it's wrong here
The router-id must be unique within the OSPF domain.
- ✓
When no 'router-id' is configured, a loopback interface with the highest IP address is preferred over a physical interface for the router-id.
Why this is correct
Loopback interfaces are preferred due to their stability.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
The 'show ip ospf' command displays the current OSPF router-id.
Why this is correct
This command shows the router-id among other OSPF details.
Related concept
Read the scenario before looking for a memorised answer.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓When no 'router-id' is configured, a loopback interface with the highest IP address is preferred over a physical interface for the router-id.Correct answer▾
Why this is correct
Loopback interfaces are preferred due to their stability.
✗The OSPF router-id is automatically derived from the MAC address of the first Ethernet interface.Wrong answer — click to see why▾
Why this is wrong here
OSPF router-id is not derived from MAC address; it is determined by the highest IP address on a loopback interface or physical interface, or manually configured.
Why candidates choose this
Students may confuse OSPF router-id selection with other protocols like OSPFv3 or EIGRP that use MAC addresses, or mistakenly think MAC is used for uniqueness.
✗If the router-id is changed using the 'router-id' command, the change takes effect immediately without any additional action.Wrong answer — click to see why▾
Why this is wrong here
Changing the router-id with the 'router-id' command does not take effect immediately; the OSPF process must be reset using 'clear ip ospf process' or the router must be reloaded.
Why candidates choose this
Students may assume that configuration changes take effect immediately, not realizing that OSPF router-id is selected at process startup and requires a reset to change.
✗The router-id must be the same on all routers in a single OSPF area.Wrong answer — click to see why▾
Why this is wrong here
Router-ids must be unique within the OSPF domain to avoid adjacency issues; they cannot be the same even in a single area.
Why candidates choose this
Students might think that within a single area, router-ids can be the same because they are not used for routing decisions, but OSPF requires uniqueness for neighbor identification.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that changing the router-id takes effect immediately, but in reality, you must clear the OSPF process or reload the router for the change to apply.
Trap categories for this question
Command / output trap
Changing the router-id requires a reload or a 'clear ip ospf process' command to take effect.
Detailed technical explanation
How to think about this question
Under the hood, OSPFv2 uses the 32-bit router-id to uniquely identify each router in the OSPF domain, and it is critical for the Link State Database (LSDB) and SPF calculations. A common real-world scenario is when an engineer configures a loopback interface after OSPF is already running; the router-id will not change until the OSPF process is restarted, which can cause temporary routing disruptions. Additionally, if two routers accidentally share the same router-id, they will fail to form an adjacency, leading to routing black holes.
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
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FAQ
Questions learners often ask
What does this 200-301 question test?
IP Routing — This question tests IP Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: When no 'router-id' is configured, a loopback interface with the highest IP address is preferred over a physical interface for the router-id. — OSPFv2 selects the router-ID based on the highest IP address of any loopback interface when no explicit 'router-id' is configured, making D correct. The 'show ip ospf' command displays the current router-ID, verifying choice E. Option A is incorrect because the router-ID is derived from IP addresses, not MAC addresses. Option B fails because changing the router-ID requires a reload or clearing the OSPF process to take effect. Option C is wrong because each router must have a unique router-ID; they do not need to match across the area.
What should I do if I get this 200-301 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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Last reviewed: Jun 11, 2026
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