CCNA IP Routing Practice Question
A router learns 172.16.0.0/16 from OSPF and 172.16.10.0/24 from a static route. Which route is used for traffic to 172.16.10.55?
⚠ Common exam trap
A frequent exam trap is assuming that the dynamic OSPF route will always be preferred over a static route, regardless of prefix length. Many candidates overlook that the router prioritizes the longest-prefix match before considering administrative distance or route source. Because 172.16.10.0/24 is more specific than 172.16.0.0/16, the router uses the static route for traffic to 172.16.10.55. Misunderstanding this can lead to incorrect answers, especially when both routes overlap. Remember, overlapping routes are common and resolved by prefix specificity, not by route type alone.
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
✓
The static /24 route
The static /24 route is used because it is more specific than the OSPF /16 route. In plain language, even though OSPF is a dynamic source and the /16 covers the destination broadly, the router prefers the entry that describes the exact destination range more precisely. Since 172.16.10.55 falls within 172.16.10.0/24, that route wins under longest-prefix match. This is a classic example of route specificity taking priority before broader route-source comparisons would matter between equal prefix lengths.
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 /16 route
Why it's wrong here
The OSPF /16 route is not used for traffic to 172.16.10.55 because routers always prioritise the longest prefix match when determining the best route. The static /24 route is more specific for the destination 172.16.10.55 than the OSPF /16 route. This option is tempting as OSPF is a dynamic protocol, often preferred for network scalability and resilience, and would be the chosen route if no more specific static route existed for that destination.
When this WOULD be correct
In a different scenario where the static route is removed or not configured, the OSPF /16 route would be the only available route for traffic to 172.16.10.55. Thus, the OSPF route would be the correct answer in that case.
- ✓
The static /24 route
Why this is correct
The static /24 route is correct because the destination 172.16.10.55 matches its prefix length of 24 bits (172.16.10.0/24), whereas the OSPF /16 route only matches the first 16 bits. Routers use longest prefix match (LPM) to select the most specific route, and the /24 is more specific than both the /16 and any default route. Even though OSPF may have a lower administrative distance, prefix length takes precedence over AD for route selection when prefixes differ, so the static /24 is installed in the forwarding table for this destination.
- ✗
The default route
Why it's wrong here
The default route is wrong because it represents 0.0.0.0/0, which is the least specific match possible. For traffic to 172.16.10.55, the router finds a longer, more specific match in the static /24 route, so the default route is not consulted. A default route is only used when no other route with a longer prefix matches the destination, regardless of the administrative distance or routing protocol priority of the default route.
When this WOULD be correct
In a different question where the router has no specific routes for the destination IP address and only a default route configured, traffic to an unknown destination would use the default route. For example, if the question stated that the router only had a default route and no other specific routes for 172.16.10.55, then the default route would be correct.
- ✗
Neither route because the prefixes overlap
Why it's wrong here
Overlapping prefixes do not disqualify both routes; the router selects the most specific match, so the /24 static route is used for 172.16.10.55. This option is tempting because overlapping routes can cause ambiguity in some routing protocols, but OSPF and static routes coexist without conflict—the longest prefix match always determines forwarding. It would be correct only if both routes had identical prefix lengths and administrative distances, creating an equal-cost path ambiguity.
When this WOULD be correct
In a scenario where a question states that both routes are configured but the static route is incorrectly defined as 172.16.0.0/24 instead of /16, the option D would be correct, as both routes would overlap and cause ambiguity in routing decisions.
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.
✓The static /24 routeCorrect answer▾
Why this is correct
The static /24 route is correct because the destination 172.16.10.55 matches its prefix length of 24 bits (172.16.10.0/24), whereas the OSPF /16 route only matches the first 16 bits. Routers use longest prefix match (LPM) to select the most specific route, and the /24 is more specific than both the /16 and any default route. Even though OSPF may have a lower administrative distance, prefix length takes precedence over AD for route selection when prefixes differ, so the static /24 is installed in the forwarding table for this destination.
✗The OSPF /16 routeWrong answer — click to see why▾
Why this is wrong here
The OSPF /16 route is less specific than the static /24 route. For traffic to 172.16.10.55, the /24 route provides a longer prefix match (24 bits vs 16 bits), so it is preferred regardless of administrative distance or routing protocol.
★ When this WOULD be the correct answer
In a different scenario where the static route is removed or not configured, the OSPF /16 route would be the only available route for traffic to 172.16.10.55. Thus, the OSPF route would be the correct answer in that case.
Why candidates choose this
Students may think OSPF is preferred over static routes due to lower administrative distance, but they forget that longest-prefix match is evaluated first. The /16 route covers the destination but is not the most specific match.
✗The default routeWrong answer — click to see why▾
Why this is wrong here
A default route (0.0.0.0/0) is only used when no more specific route matches the destination. Since a matching /24 route exists for 172.16.10.55, the default route is not considered.
★ When this WOULD be the correct answer
In a different question where the router has no specific routes for the destination IP address and only a default route configured, traffic to an unknown destination would use the default route. For example, if the question stated that the router only had a default route and no other specific routes for 172.16.10.55, then the default route would be correct.
Why candidates choose this
Students might think that if a route is learned via OSPF and a static route exists, the router might fall back to a default route. However, the presence of any matching route (even a static one) prevents the default route from being used.
✗Neither route because the prefixes overlapWrong answer — click to see why▾
Why this is wrong here
Overlapping routes are common in routing tables and are resolved by the longest-prefix match algorithm. The router selects the most specific route (the /24) for the destination, so there is no conflict or ambiguity.
★ When this WOULD be the correct answer
In a scenario where a question states that both routes are configured but the static route is incorrectly defined as 172.16.0.0/24 instead of /16, the option D would be correct, as both routes would overlap and cause ambiguity in routing decisions.
Why candidates choose this
Some students may incorrectly believe that overlapping prefixes cause a routing loop or error, but routers handle overlapping routes by choosing the most specific match. This is a fundamental concept in IP routing.
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
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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.
Key term
Static route
A static route is a manually configured path in a router's routing table that tells the router exactly where to send packets for a specific destination network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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