CCNA IP Routing Practice Question
A router receives a destination prefix from EIGRP with administrative distance 90 and also from OSPF with administrative distance 110. The prefix length is identical. Which route source is preferred?
⚠ Common exam trap
A frequent exam trap is to assume that OSPF routes might be preferred over EIGRP routes simply because OSPF is a widely used IGP or because of metric comparisons within OSPF. Candidates often confuse administrative distance with routing metrics, mistakenly thinking that the lower metric route is preferred regardless of protocol. However, administrative distance is the first criterion when routes come from different protocols. Another trap is to think that routes from different protocols cannot overlap or that prefix length alone determines preference. The key is that when prefix lengths are equal, the router uses administrative distance to select the best route, so EIGRP’s lower AD of 90 always beats OSPF’s 110.
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
✓
EIGRP
The EIGRP route is preferred because its administrative distance is lower. In practical terms, once the prefix length is the same, the router compares the trustworthiness of the route source. Lower administrative distance wins. Since 90 is lower than 110, EIGRP is preferred over OSPF for that destination. This is an administrative-distance comparison question, not a longest-prefix question. The key is that the prefix length is equal, so source preference becomes the deciding factor.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
EIGRP
Why this is correct
EIGRP is the correct choice because its default administrative distance of 90 is lower than OSPF's default of 110 for the same destination prefix. When a router receives overlapping routes from multiple routing protocols, the protocol with the lowest administrative distance is deemed most trustworthy and installed in the routing table. The EIGRP route becomes the active route, while the OSPF route is kept as a backup or not installed at all.
- ✗
OSPF
Why it's wrong here
OSPF would not be selected because its default administrative distance is 110, which is higher than EIGRP's 90. Even if OSPF's metric were extremely low, the router compares administrative distance first when the routes come from different protocols. Since OSPF's distance is greater, its route cannot be preferred over EIGRP's, so choosing OSPF is incorrect.
When this WOULD be correct
In a different question setup, if the administrative distance for OSPF were lower than that of EIGRP (e.g., OSPF 90 and EIGRP 110), then OSPF would be the preferred route source for the identical prefix length.
- ✗
Both equally
Why it's wrong here
EIGRP and OSPF cannot treat the same prefix with equal preference because the router's decision process uses administrative distance as a tie-breaker for routes learned from different protocols. EIGRP holds a default administrative distance of 90, while OSPF uses 110, so the numerically lower value is always selected. Since the distances differ, the route sources are not considered equal, making this option incorrect.
When this WOULD be correct
In a different question setup where both EIGRP and OSPF provide routes with the same administrative distance and prefix length, the candidate might be asked which routes would be preferred. In that case, the correct answer could be 'both equally' as they would be treated with the same preference.
- ✗
Neither, because route sources cannot overlap
Why it's wrong here
This option is wrong because route sources for the same prefix absolutely can overlap; a router can run both EIGRP and OSPF and learn the same destination network from each. In fact, administrative distance exists specifically to resolve such overlaps by assigning trust levels to different protocols. The router compares the distances and picks the lower one, so saying the sources cannot overlap and therefore neither is chosen misunderstands the routing logic.
When this WOULD be correct
In a scenario where the question specifies that a router cannot use routes from multiple sources for the same destination prefix, such as in a strict routing policy or a specific network design requirement, this option would be correct.
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.
✓EIGRPCorrect answer▾
Why this is correct
EIGRP is the correct choice because its default administrative distance of 90 is lower than OSPF's default of 110 for the same destination prefix. When a router receives overlapping routes from multiple routing protocols, the protocol with the lowest administrative distance is deemed most trustworthy and installed in the routing table. The EIGRP route becomes the active route, while the OSPF route is kept as a backup or not installed at all.
✗OSPFWrong answer — click to see why▾
Why this is wrong here
OSPF has a default administrative distance of 110, which is higher than EIGRP's 90. Therefore, OSPF is not preferred when both protocols provide a route to the same destination prefix.
★ When this WOULD be the correct answer
In a different question setup, if the administrative distance for OSPF were lower than that of EIGRP (e.g., OSPF 90 and EIGRP 110), then OSPF would be the preferred route source for the identical prefix length.
Why candidates choose this
Students might think OSPF is more reliable or commonly used in larger networks, leading them to assume it would be preferred over EIGRP, but administrative distance is the decisive factor.
✗Both equallyWrong answer — click to see why▾
Why this is wrong here
When two routing protocols provide routes to the same prefix, the router selects the one with the lower administrative distance. Since EIGRP's AD (90) is lower than OSPF's (110), they are not equally preferred.
★ When this WOULD be the correct answer
In a different question setup where both EIGRP and OSPF provide routes with the same administrative distance and prefix length, the candidate might be asked which routes would be preferred. In that case, the correct answer could be 'both equally' as they would be treated with the same preference.
Why candidates choose this
A student might confuse administrative distance with metric, thinking that if both routes have the same metric they would be equal, but AD is compared first and differs here.
✗Neither, because route sources cannot overlapWrong answer — click to see why▾
Why this is wrong here
Routers can receive the same prefix from multiple routing protocols and will compare administrative distances to choose the best route. This is a common scenario in networks running multiple protocols, so route sources can overlap.
★ When this WOULD be the correct answer
In a scenario where the question specifies that a router cannot use routes from multiple sources for the same destination prefix, such as in a strict routing policy or a specific network design requirement, this option would be correct.
Why candidates choose this
A student might think that different routing protocols cannot both know about the same network, but in practice, route redistribution or multiple protocols on the same router can lead to overlapping prefixes.
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
Interpreting HSRP and VRRP Operational Status
Key term
Prefix length
A prefix length is the number of bits in a subnet mask that identifies the network portion of an IP address, written after a slash (e.g., /24).
Key term
Enhanced Interior Gateway Routing Protocol
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary advanced distance-vector routing protocol that calculates the best path through a network using multiple metrics, with fast convergence and loop-free routing.
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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.