Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Which statement best explains why a router uses longest-prefix match before other route-comparison steps between matching routes?
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
Best answer
Because the router first prefers the route that most precisely matches the destination.
This is correct because longest-prefix match is based on specificity.
B
Distractor review
Because administrative distance is meaningful only for MAC addresses.
This is wrong because administrative distance is a route-source trust concept, not a MAC-address concept.
C
Distractor review
Because default routes are considered more specific than host routes.
This is wrong because default routes are the least specific, not the most specific.
D
Distractor review
Because OSPF requires it only on Ethernet links.
This is wrong because longest-prefix match is general routing logic, not an Ethernet-only OSPF rule.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is confusing longest-prefix match with administrative distance or metric as the first step in route selection. Candidates may incorrectly believe that administrative distance, which ranks route trustworthiness, is applied before prefix length. Another common mistake is thinking default routes are more specific than host routes, when in fact default routes have the shortest prefix and are the least specific. Misunderstanding these concepts can lead to incorrect answers about routing behavior. Remember, longest-prefix match always precedes other route-comparison steps to ensure the router selects the most precise route before considering other factors.
Technical deep dive
How to think about this question
Longest-prefix match is a fundamental routing principle where a router selects the route with the most specific subnet mask that matches the destination IP address. This means the route with the longest subnet prefix length (most bits set to 1) is preferred because it narrows down the destination network more precisely than broader routes. This specificity ensures that packets are forwarded along the most accurate path possible, reducing ambiguity in routing decisions. When multiple routes to the same destination exist, the router first applies the longest-prefix match rule to filter out less specific routes. Only after identifying routes with the same prefix length does the router compare other attributes such as administrative distance and metric to determine the best path. This hierarchical decision process guarantees that route specificity takes precedence, which is critical in complex networks where overlapping routes may exist. A common exam trap is misunderstanding the role of longest-prefix match versus administrative distance or metric. Some candidates incorrectly assume that administrative distance is the first criterion or that default routes are more specific than host routes. In practice, default routes have the shortest prefix (0.0.0.0/0) and are the least specific. Cisco routers always apply longest-prefix match first to ensure the most precise route is chosen before considering trustworthiness or cost metrics. This behavior is consistent across routing protocols and is essential for predictable routing outcomes.
KKey Concepts to Remember
- A router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
- Longest-prefix match ensures packets are forwarded along the most precise path by preferring routes with longer subnet prefixes.
- When multiple routes match a destination, the router first filters by prefix length before comparing administrative distance or metrics.
- Default routes have the shortest prefix length and are considered the least specific routes in routing decisions.
- Administrative distance is used only after longest-prefix match to choose between routes with equal prefix lengths.
- Routing protocols like OSPF and EIGRP rely on longest-prefix match as a fundamental routing table lookup rule.
- Ignoring longest-prefix match would cause routers to select less specific routes, reducing routing accuracy and efficiency.
- Longest-prefix match is a universal routing principle applied regardless of the underlying routing protocol or interface type.
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 router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
What is the correct answer to this question?
The correct answer is: Because the router first prefers the route that most precisely matches the destination. — Longest-prefix match comes first because route specificity is the most direct way to identify the best matching destination range. In plain language, if several routes could all technically match a packet, the router first asks which one describes the destination most precisely. Only after that does it compare other properties among routes of equal prefix length. This is fundamental to routing-table logic. If the router ignored prefix specificity, detailed route entries would lose much of their value. The correct answer is the one that explains route choice in terms of the most specific valid match.
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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