CCNA Network Infrastructure and Connectivity Practice Question
Which rule does a router apply first when selecting a route for a destination packet?
⚠ Common exam trap
Remember, prefix length is the primary factor in route selection, not administrative distance or metrics.
Answer choices
Why each option matters
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.
Answer analysis
Option-by-option breakdown
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 only meaningful when comparing routes learned by the same routing protocol; combining OSPF cost and EIGRP composite metric is invalid. Additionally, before any metric comparison can occur, the router must have completed the longest prefix match for the specific destination network. Across different protocols, the router first applies administrative distance to pick a protocol winner, and only then does the lowest metric within that selected protocol matter.
When this WOULD be correct
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.
- ✗
Oldest route in the routing table
Why it's wrong here
Route age, which records how long a route has remained stable in the routing table, is not a criterion in the packet-forwarding path. The data plane performs a longest-prefix-match lookup in the forwarding table without considering age; only routing protocols may optionally use age for route convergence or staleness decisions. Therefore, an older route cannot be selected ahead of a newer route, regardless of any preference on aging.
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
A router's IP forwarding logic first identifies all routes whose prefix matches the destination address's network portion. It then selects the route with the longest subnet mask because that prefix is the most specific representation of the destination, even if that route has a higher metric. This rule is non-negotiable and precedes any metric or administrative-distance comparison.
- ✗
Default route if one exists
Why it's wrong here
The default route, 0.0.0.0/0, has a prefix length of zero and therefore is the least specific route in the routing table. A router always prefers a route with a longer prefix length, such as a /24 or even /1, because that route is more precise and better matches the destination. The default route is consulted only after all other more-specific matches fail, making it the last choice, not the first.
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
Why each answer is right or wrong
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.
✓Longest prefix matchCorrect answer▾
Why this is correct
A router's IP forwarding logic first identifies all routes whose prefix matches the destination address's network portion. It then selects the route with the longest subnet mask because that prefix is the most specific representation of the destination, even if that route has a higher metric. This rule is non-negotiable and precedes any metric or administrative-distance comparison.
✗Lowest metric across all protocolsWrong answer — click to see why▾
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.
✗Oldest route in the routing tableWrong answer — click to see why▾
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.
✗Default route if one existsWrong answer — click to see why▾
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?”
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
Default Routes
Key term
Route
A route is a path that data takes through a network from one device or network to another, determined by routing protocols and configured rules.
Key term
Prefix length
A prefix length is the number of bits in a subnet mask that identifies the network portion of an IP address, written after a slash (e.g., /24).
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.