Quick Answer
The correct answer is longest prefix match, because a router’s primary duty is to forward packets to the most specific destination network available. When a packet arrives, the router examines its routing table for all routes that match the destination IP, then selects the entry with the longest subnet mask—this ensures the packet follows the most precise path, even if a less specific route has a better administrative distance or metric. On the CCNA 200-301 v2 exam, this concept often appears in questions about route selection order, where many students mistakenly choose AD or metric first. The common trap is forgetting that prefix length is checked before any other tiebreaker; AD and metric only come into play when two routes share the exact same prefix length. A solid memory tip is “longest match first, then trust, then cost”—prefix length always wins the first round.
This 200-301 practice question tests your understanding of network infrastructure and connectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: a Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which rule does a router apply first when selecting a route for a destination packet?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Lowest metric across all protocols
Why wrong: Metrics are not compared before longest prefix match across unrelated routes.
Oldest route in the routing table
Why wrong: Route age is not the first selection rule.
Longest prefix match
Correct. Most-specific route wins first.
Default route if one exists
Why wrong: A default route is used only when no more specific route matches.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Longest prefix match
Routers first look for the most specific matching prefix. Administrative distance and metrics matter when competing routes exist for the same destination prefix length.
Key principle: A Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Lowest metric across all protocols
Why it's wrong here
Metrics are not compared before longest prefix match across unrelated routes.
Oldest route in the routing table
Why it's wrong here
Route age is not the first selection rule.
When this WOULD be correct
If the exam question asked which route a router would choose when all other metrics are equal, and the only differentiating factor is the age of the routes, then selecting the oldest route could be considered correct in that specific context.
Longest prefix match
Why this is correct
Correct. Most-specific route wins first.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
A Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet.
Default route if one exists
Why it's wrong here
A default route is used only when no more specific route matches.
When this WOULD be correct
If the exam question asked which route a router would select when no specific routes exist for a destination, then option D would be correct. In this scenario, the router would use the default route as a fallback to forward packets.
Option-by-option analysis
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.
Why this is correct
Correct. Most-specific route wins first.
Why this is wrong here
The router first uses the longest prefix match to select a route; metrics are only compared among routes from the same routing protocol or when administrative distance is equal. Comparing metrics across different protocols is not the first step.
★ When this WOULD be the correct answer
In a question focused on route selection across multiple routing protocols where the lowest metric is explicitly defined as the primary selection criterion, such as in a scenario comparing OSPF and EIGRP routes, this option would be correct.
Why candidates choose this
Students often confuse the order of route selection, thinking that lower metric is always preferred, but metric comparison occurs only after the longest prefix match and administrative distance checks.
Why this is wrong here
Route age is not a primary selection criterion; routers use longest prefix match first, then administrative distance, then metric. Older routes are not preferred over newer ones in the selection process.
★ When this WOULD be the correct answer
If the exam question asked which route a router would choose when all other metrics are equal, and the only differentiating factor is the age of the routes, then selecting the oldest route could be considered correct in that specific context.
Why candidates choose this
Some might think that older routes are more stable or trusted, but route age is irrelevant for forwarding decisions; it is only used in some routing protocols for tie-breaking (e.g., OSPF) but not as the first rule.
Why this is wrong here
A default route is only used when no other route matches the destination; the router first checks for more specific matches using the longest prefix match. The default route is the least preferred.
★ When this WOULD be the correct answer
If the exam question asked which route a router would select when no specific routes exist for a destination, then option D would be correct. In this scenario, the router would use the default route as a fallback to forward packets.
Why candidates choose this
Students may think that a default route is a catch-all and thus applied first, but in reality, it is the last resort after all other routes have been evaluated.
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?”
Common exam traps
Remember, prefix length is the primary factor in route selection, not administrative distance or metrics.
Detailed technical explanation
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
Key takeaway
A Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Review a Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — A Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet..
The correct answer is: Longest prefix match — Routers first look for the most specific matching prefix. Administrative distance and metrics matter when competing routes exist for the same destination prefix length.
Review a Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet., then practise related 200-301 questions on the same topic to reinforce the concept.
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
A Cisco router selects the route with the longest prefix match as the first step in forwarding a destination packet.
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Last reviewed: May 17, 2026
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