Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
The OSPF /16 route
This is wrong because the /16 is less specific than the matching /24 route.
B
Best answer
The static /24 route
This is correct because 172.16.10.55 falls within the more specific /24 route.
C
Distractor review
The default route
This is wrong because a specific matching route exists and is preferred.
D
Distractor review
Neither route because the prefixes overlap
This is wrong because overlapping routes are normal and longest-prefix match resolves the choice.
Common exam trap
Common exam trap: answer the scenario, not the keyword
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.
Technical deep dive
How to think about this question
Routing decisions in Cisco routers rely heavily on the concept of longest-prefix match, where the most specific route to a destination IP address is preferred over less specific ones. In this scenario, the router has learned two routes for the 172.16.0.0 network: a /16 prefix from OSPF and a more specific /24 prefix from a static route. The /24 subnet mask covers a smaller, more precise range of IP addresses, including 172.16.10.55, making it the preferred route for traffic destined to that IP. The router first matches the destination IP against all known routes and selects the route with the longest subnet mask (most bits matched). Even though OSPF is a dynamic routing protocol and static routes typically have a lower administrative distance, the key factor here is prefix length. The static /24 route is more specific than the OSPF /16 route, so the router uses the static route for forwarding packets to 172.16.10.55. Administrative distance would only come into play if two routes had the same prefix length. A common exam trap is confusing route preference with administrative distance or the routing protocol source. Candidates might incorrectly assume OSPF routes always override static routes or that overlapping prefixes cause routing conflicts. In reality, overlapping prefixes are normal, and the router always uses the longest-prefix match rule first. This behavior ensures precise routing and efficient traffic forwarding in complex networks, reflecting real-world Cisco routing operations.
KKey Concepts to Remember
- A Cisco router uses the longest-prefix match rule to select the most specific route to forward traffic to a destination IP address.
- Static routes with a more specific subnet mask are preferred over less specific dynamic routes when both cover the destination IP.
- OSPF advertises routes dynamically but does not override a static route if the static route has a longer prefix length.
- Administrative distance is only compared when two routes have the same prefix length to the same destination network.
- Overlapping IP prefixes are common in routing tables and are resolved by choosing the route with the longest subnet mask.
- The router forwards packets based on the route entry that precisely matches the destination IP address range.
- Static routes are manually configured and can provide more specific routing entries than dynamically learned routes.
- Understanding prefix length and longest-prefix match is critical for accurate routing decisions in Cisco networks.
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.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
A Cisco router uses the longest-prefix match rule to select the most specific route to forward traffic to a destination IP address.
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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