Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1# show ip route O 172.16.10.0/24 [110/20] via 10.1.1.2 S 172.16.10.128/25 [1/0] via 10.1.1.6 O 172.16.0.0/16 [110/30] via 10.1.1.10 S* 0.0.0.0/0 [1/0] via 203.0.113.1
A router has these routes installed. Which path will be chosen for traffic to 172.16.10.200?
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
O 172.16.10.0/24 via 10.1.1.2
This is wrong because the /24 matches, but the /25 is more specific and also matches the destination.
B
Best answer
S 172.16.10.128/25 via 10.1.1.6
This is correct because 172.16.10.200 falls within the 172.16.10.128/25 range, making it the most specific matching route.
C
Distractor review
O 172.16.0.0/16 via 10.1.1.10
This is wrong because the /16 is broader than both the /24 and the /25 and will not be preferred when a more specific match exists.
D
Distractor review
S* 0.0.0.0/0 via 203.0.113.1
This is wrong because a default route is used only when there is no more specific match available.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is to select a route based solely on the routing protocol type or administrative distance without verifying the subnet mask specificity. For example, candidates might incorrectly choose the OSPF-learned /24 route over the static /25 route because they assume dynamic routes are preferred or because they overlook the subnet boundaries. This mistake ignores the fundamental longest-prefix match rule, which always prioritizes the most specific subnet regardless of protocol. Misreading subnet ranges or confusing the address boundaries of a /25 versus a /24 can also lead to choosing an incorrect route.
Technical deep dive
How to think about this question
Routing decisions in Cisco routers rely heavily on the concept of longest-prefix match, which means the router selects the route with the most specific subnet mask that includes the destination IP address. In this scenario, the router has multiple routes to the 172.16.10.0 network with different subnet masks: a /16, a /24, and a /25. The /25 subnet is the most specific because it covers a smaller range of IP addresses, allowing the router to forward traffic more precisely. When a packet destined for 172.16.10.200 arrives, the router evaluates each route to see which subnet contains this IP. The /25 subnet 172.16.10.128/25 covers IP addresses from 172.16.10.128 to 172.16.10.255, which includes 172.16.10.200. Although the /24 and /16 routes also cover this address, the router prefers the /25 route due to its longer prefix length, ensuring more accurate routing. This behavior is fundamental in Cisco routing and is consistent across routing protocols and static routes. A common exam trap is to overlook the subnet boundaries and assume that a broader subnet like /24 or /16 will be chosen simply because it appears first or is learned via a routing protocol like OSPF. However, Cisco routers always prioritize the most specific route regardless of the routing protocol or administrative distance when multiple matches exist. Practically, this ensures traffic is routed optimally and prevents suboptimal routing paths that could cause inefficiencies or routing loops.
KKey Concepts to Remember
- A Cisco router selects the route with the longest prefix match to forward traffic to a destination IP address.
- Subnet masks with more bits set (longer prefixes) represent more specific routes and are preferred over broader subnets.
- Static routes (S) and OSPF routes (O) are compared based on prefix length first before considering administrative distance or metric.
- The router includes a destination IP in a subnet if the IP falls within the subnet's address range defined by the prefix length.
- Default routes (0.0.0.0/0) are only used when no more specific route matches the destination IP address.
- Routing protocols advertise routes with varying prefix lengths, but the router always prefers the most specific matching route.
- Ignoring subnet boundaries can lead to selecting less optimal routes, which is a common exam trap in routing questions.
- The longest prefix match rule ensures efficient and accurate routing by narrowing down the forwarding path to the smallest subnet.
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.
Related practice questions
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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 selects the route with the longest prefix match to forward traffic to a destination IP address.
What is the correct answer to this question?
The correct answer is: S 172.16.10.128/25 via 10.1.1.6 — When multiple routes could match a destination, the router first uses longest-prefix match. In plain language, it chooses the route that describes the destination most specifically, not simply the one from the protocol with the lowest metric somewhere else in the table. For 172.16.10.200, the key is to determine which listed prefix actually contains that address and is the narrowest valid match. If a /25 and a /24 both match, the /25 wins because it is more specific. If the destination does not fit inside the smaller subnet, then the broader route is used instead. That is the entire skill being tested here: careful evaluation of the prefix boundaries before thinking about anything else. Once you recognize the address falls within the upper half of the /24, the more specific route becomes obvious.
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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