CCNA Network Infrastructure and Connectivity Practice Question
At which OSI layer do routers make forwarding decisions based on logical addressing?
⚠ Common exam trap
Don't confuse the roles of routers and switches. Remember that routers use logical addressing (IP addresses) at Layer 3, while switches use physical addressing (MAC addresses) at Layer 2.
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
✓
Layer 3
Routers operate at the network layer when making forwarding decisions based on logical Layer 3 addresses such as IPv4 or IPv6 destination addresses.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Layer 1
Why it's wrong here
Layer 1, the physical layer, only defines the electrical/optical signaling, cable types, and bit-level synchronization needed to transmit raw data over a medium. A router may receive and transmit bits through its physical interfaces at this layer, but it makes no forwarding decisions here because Layer 1 has no concept of addressing or network topology.
When this WOULD be correct
If the question were to ask about the physical aspects of network communication, such as how signals are transmitted over cables or wireless media, then Layer 1 would be the correct answer.
- ✗
Layer 2
Why it's wrong here
Layer 2, the data link layer, switches forward frames based on destination MAC addresses and keep a MAC address table for the local LAN segment. Routers are Layer 3 devices, but they do use ARP to resolve the MAC address of the next-hop IP; however, the actual routing decision is based on the destination IP address at the network layer, not on the Layer 2 address.
When this WOULD be correct
If the question asked about devices that operate at Layer 2, such as switches making decisions based on MAC addresses, then option B would be correct. An example question could be: 'At which OSI layer do switches make forwarding decisions based on physical addressing?'
- ✓
Layer 3
Why this is correct
Routers make forwarding decisions at OSI Layer 3, the network layer, by inspecting the destination IPv4 or IPv6 address in the packet header. They consult their routing table and apply the longest-prefix-match rule to select the next hop toward the destination network. This logical addressing allows routing across different broadcast domains.
- ✗
Layer 4
Why it's wrong here
Layer 4, the transport layer, manages end-to-end communication through protocols such as TCP and UDP, using port numbers, sequence numbers, and acknowledgments for reliability and data segmentation. Routers performing standard unicast forwarding do not read transport-layer headers; they ignore port numbers and treat each packet as a network-layer datagram, so transport semantics do not determine the next hop.
When this WOULD be correct
In a question that asks about the role of Layer 4 in establishing connections and managing data transfer reliability, such as 'At which OSI layer do devices manage session establishment and data integrity?', option D would be correct.
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.
✓Layer 3Correct answer▾
Why this is correct
Routers make forwarding decisions at OSI Layer 3, the network layer, by inspecting the destination IPv4 or IPv6 address in the packet header. They consult their routing table and apply the longest-prefix-match rule to select the next hop toward the destination network. This logical addressing allows routing across different broadcast domains.
✗Layer 1Wrong answer — click to see why▾
Why this is wrong here
Layer 1 (Physical layer) deals with the physical transmission of bits over media, such as cables or radio frequencies. Routers do not make forwarding decisions at this layer; they only handle electrical or optical signals.
★ When this WOULD be the correct answer
If the question were to ask about the physical aspects of network communication, such as how signals are transmitted over cables or wireless media, then Layer 1 would be the correct answer.
Why candidates choose this
Students might confuse the physical connectivity of routers (e.g., interfaces, cables) with the layer at which routing decisions occur, but forwarding decisions are not made at Layer 1.
✗Layer 2Wrong answer — click to see why▾
Why this is wrong here
Layer 2 (Data Link layer) uses MAC addresses for switching within the same network segment. Routers, however, forward packets based on Layer 3 logical addresses, not MAC addresses, which are only used for next-hop delivery.
★ When this WOULD be the correct answer
If the question asked about devices that operate at Layer 2, such as switches making decisions based on MAC addresses, then option B would be correct. An example question could be: 'At which OSI layer do switches make forwarding decisions based on physical addressing?'
Why candidates choose this
Since routers have MAC addresses and use ARP, some may mistakenly think routing occurs at Layer 2. However, routing decisions are based on logical addressing, not MAC addresses.
✗Layer 4Wrong answer — click to see why▾
Why this is wrong here
Layer 4 (Transport layer) handles end-to-end communication, segmentation, and flow control using protocols like TCP and UDP. Routers do not use Layer 4 information for forwarding decisions; they only examine Layer 3 headers.
★ When this WOULD be the correct answer
In a question that asks about the role of Layer 4 in establishing connections and managing data transfer reliability, such as 'At which OSI layer do devices manage session establishment and data integrity?', option D would be correct.
Why candidates choose this
Students might associate routers with port numbers or stateful inspection (e.g., ACLs) and think Layer 4 is involved, but basic routing is purely Layer 3.
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
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Key term
Internet Protocol
Internet Protocol (IP) is the set of rules that governs how data is addressed, routed, and sent from one device to another across networks, including the internet.
Key term
IPv6
IPv6 is the most recent version of the Internet Protocol, designed to replace IPv4 by providing a vastly larger number of unique addresses and improved network features.
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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.