CCNA IP Routing Practice Question
A network engineer adds a loopback interface Lo0 with IP address 172.16.0.1/32 to router R1. After restarting the OSPF process, the OSPF router-ID changes from 10.1.1.1 to 172.16.0.1, and the neighbor relationship with R2 resets. What should the technician do next to prevent this disruption the next time a loopback is added?
⚠ Common exam trap
Cisco often tests the misconception that adding a loopback interface automatically changes the OSPF router-ID immediately, but the trap here is that the router-ID only changes after an OSPF process restart or router reload, and candidates may incorrectly think passive-interface or priority settings can prevent the disruption.
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
✓
Configure a static router-id using the router-id command under the OSPF process
The OSPF router-ID is dynamically selected based on the highest IP address of any loopback interface, or if none exist, the highest IP of any physical interface. Adding a new loopback with a higher IP than the current router-ID causes OSPF to reselect a new router-ID upon process restart, which resets all neighbor adjacencies. Configuring a static router-ID with the 'router-id' command under the OSPF process prevents this disruption by fixing the router-ID regardless of interface IP changes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure passive-interface Lo0 under the OSPF process
Why it's wrong here
Passive-interface only suppresses OSPF hello packets on the loopback; it does not prevent the loopback IP from being used in router-ID election. The highest loopback IP will still become the router-ID at next process restart.
- ✓
Configure a static router-id using the router-id command under the OSPF process
Why this is correct
The OSPF router-ID is dynamically derived from the highest loopback or active interface IP unless overridden. Creating another loopback with a numerically higher address would cause OSPF to adopt it as the new router-ID at the next process restart, tearing down every existing neighbor relationship. The `router-id` command forces the router to retain that fixed value indefinitely, because manually configured router-IDs take precedence over all IPv4/IPv6 interface addresses, making the network stable regardless of later loopback additions.
- ✗
Set a higher OSPF priority on the router’s interfaces
Why it's wrong here
Interface priority is a parameter exchanged in OSPF hello packets and is considered only during Designated Router and Backup Designated Router elections on a shared multi-access segment. Router-ID selection is a separate mechanism that is decided from loopback or physical interface IPv4 addresses, or from an explicit `router-id` configuration, and it is completely independent of priority values. Raising priority may change which neighboring router becomes the DR for that subnet, but it cannot stop the local router from re-selecting a different router-ID and resetting its adjacencies.
- ✗
Configure the OSPF area as a stub area
Why it's wrong here
Configuring the OSPF area as a stub changes the types of LSAs that are permitted inside that area, primarily blocking Type-5 external LSAs and forcing the use of a default route for external destinations. This LSA filtering has no relation to router-ID election, which is a per-router property computed before any routing exchange even begins. A router-ID change still triggers the same OSPF neighbor state machine resets regardless of the area type, so the stub configuration neither masks nor remedies the adjacency instability described.
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.
✓Configure a static router-id using the router-id command under the OSPF processCorrect answer▾
Why this is correct
The OSPF router-ID is dynamically derived from the highest loopback or active interface IP unless overridden. Creating another loopback with a numerically higher address would cause OSPF to adopt it as the new router-ID at the next process restart, tearing down every existing neighbor relationship. The `router-id` command forces the router to retain that fixed value indefinitely, because manually configured router-IDs take precedence over all IPv4/IPv6 interface addresses, making the network stable regardless of later loopback additions.
✗Configure passive-interface Lo0 under the OSPF processWrong answer — click to see why▾
Why this is wrong here
Passive-interface does not influence router-ID selection, which is based solely on highest active loopback IP address at process initialization.
✗Set a higher OSPF priority on the router’s interfacesWrong answer — click to see why▾
Why this is wrong here
Confusing DR election parameters with router-ID election leads candidates to a parameter that is irrelevant to router-ID stability.
✗Configure the OSPF area as a stub areaWrong answer — click to see why▾
Why this is wrong here
Changing area type does not address router-ID fluctuation; it targets LSDB optimization, which is unrelated to the dynamic router-ID re-election after loopback addition.
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
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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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JA
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.