Question 1mediummulti select
Full question →mediummulti selectObjective-mapped
Which two statements accurately describe longest-prefix match in routing?
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
The most specific matching route is preferred over broader matching routes.
This is correct because longest-prefix match is based on route specificity.
B
Distractor review
A default route is preferred over a matching /24 because it is simpler.
This is wrong because the default route is less specific and is used only when no better match exists.
C
Best answer
Longest-prefix match is evaluated before choosing between broader and narrower matching routes.
This is correct because specificity is the first key factor among matching prefixes.
D
Distractor review
Routers ignore subnet masks and choose only by administrative distance.
This is wrong because prefix length is central to longest-prefix match.
E
Distractor review
A /16 is always more specific than a /24.
This is wrong because /24 is more specific than /16.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that a default route or a route with a simpler prefix is preferred over a more specific route. Some candidates mistakenly believe that administrative distance or metric always takes precedence over prefix length. However, Cisco routers always apply longest-prefix match first, selecting the most specific route before considering administrative distance or metric. This misunderstanding can lead to incorrect answers when default routes or broader prefixes appear in the routing table alongside more specific routes.
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 that among multiple routes that could forward a packet, the one with the longest subnet prefix (highest number of matching bits) is preferred. This concept ensures precise routing decisions, allowing routers to forward packets to the most narrowly defined network segment possible. When a router receives a packet, it compares the destination IP address against all routes in its routing table. The router first filters routes by prefix length, choosing the longest prefix match before considering other factors such as administrative distance or metric. Only after identifying routes with the same prefix length does the router evaluate these secondary criteria to select the best path. This prioritization guarantees that specificity in subnetting takes precedence in routing decisions. A common exam trap is misunderstanding the role of default routes and administrative distance in longest-prefix match. While administrative distance and metrics influence route selection, they only apply after the router identifies the longest prefix matches. Default routes (with prefix /0) are the least specific and are only used if no more specific route exists. Practically, this behavior ensures routers forward packets efficiently by narrowing down the destination as much as possible before falling back to broader routes.
KKey Concepts to Remember
- Longest-prefix match means a router prefers the route with the most specific subnet mask that matches the destination IP address.
- Routers compare prefix lengths first and select the route with the longest matching prefix before considering administrative distance or metric.
- A default route is the least specific and is only used when no other more specific route matches the destination address.
- Administrative distance and metric are secondary criteria evaluated only after the router identifies routes with the same prefix length.
- A route with a /24 prefix is more specific than a /16 prefix because it matches more bits of the destination IP address.
- Longest-prefix match ensures precise routing by narrowing the destination range to the smallest possible subnet.
- Routers do not ignore subnet masks; prefix length is central to determining the best route in the routing table.
- Choosing the longest prefix match prevents routing loops and ensures efficient packet forwarding in complex 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?
Longest-prefix match means a router prefers 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: The most specific matching route is preferred over broader matching routes. — Longest-prefix match means a router prefers the most specific route that matches the destination. In plain language, if several entries could apply to a packet, the router chooses the one that narrows the destination range most precisely. This is why a /25 can beat a /24, and a /24 can beat a /16, even if all three technically contain the same destination address. The common mistake is to assume the router always begins with protocol trust or metric. Those factors matter, but only after the router has determined which matching routes share the same prefix length.
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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