CCNA IP Routing Practice Question
Which TWO statements correctly describe OSPFv2 router-id selection and verification in a single-area configuration?
⚠ Common exam trap
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.
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.
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 OSPF router ID is a 32-bit field that is represented in IP address format, but it is not derived from the MAC address of an interface. Instead, the router ID is either manually configured or automatically selected based on the highest IP address among loopback interfaces, then the highest IP address among physical interfaces. MAC addresses operate at Layer 2 and have no role in OSPF router ID selection, so this statement is false.
- ✗
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
The 'router-id' command changes the configured OSPF router ID, but the OSPF process selects the router ID at startup and continues using the old value until the process is restarted. To see the change take effect, you must either reload the router or issue the 'clear ip ospf process' command, which rebuilds all OSPF adjacencies and re-selects the router ID. Therefore, saying the change is immediate is incorrect.
- ✗
The router-id must be the same on all routers in a single OSPF area.
Why it's wrong here
The OSPF router ID must be unique across the entire OSPF routing domain, not necessarily the same within an area. If two routers in the same OSPF domain share a router ID, the OSPF adjacency can fail to establish, and LSAs can be incorrectly identified, causing routing instability. In fact, routers in different areas must also have unique router IDs because the router ID is used to identify the originator of LSA information domain-wide.
- ✓
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
When no explicit 'router-id' command is configured, OSPF determines the router ID by first preferring the IP address of any loopback interface, choosing the highest numeric loopback address, and only then falling back to the highest IP address on a physical interface. Loopback interfaces are always up and do not depend on physical link state, making them stable and thus preferred for router ID selection. This behavior is defined by the OSPF RFC and is a well-known practical rule.
- ✓
The 'show ip ospf' command displays the current OSPF router-id.
Why this is correct
The 'show ip ospf' command displays detailed OSPF process information, including the current OSPF router ID, the router's OSPF process ID, router type, SPF schedule and timer values, and area information. This command is a standard verification tool used to confirm which router ID OSPF is actually using—especially useful after a router ID change or when verifying loopback priority. It does not require enable mode and is often one of the first troubleshooting commands an engineer runs.
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
When no explicit 'router-id' command is configured, OSPF determines the router ID by first preferring the IP address of any loopback interface, choosing the highest numeric loopback address, and only then falling back to the highest IP address on a physical interface. Loopback interfaces are always up and do not depend on physical link state, making them stable and thus preferred for router ID selection. This behavior is defined by the OSPF RFC and is a well-known practical rule.
✗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?”
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.
Go deeper
Related to this question
Learn chapter
OSPFv3 Single-Area Configuration for IPv6
Key term
IP address
An IP address is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.