200-901 Network Fundamentals Practice Question
A network engineer is troubleshooting connectivity issues and wants to verify the path that packets take from a source to a destination IP address. Which OSI layer is primarily responsible for packet forwarding and routing?
⚠ Common exam trap
Cisco often tests the distinction between Layer 2 switching (MAC-based forwarding within a LAN) and Layer 3 routing (IP-based forwarding between networks), and the trap here is that candidates confuse the Data Link layer's local forwarding with the Network layer's path determination.
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 - Network
The Network layer (Layer 3) is responsible for packet forwarding and routing, using logical IP addresses to determine the best path from source to destination. Protocols like IP (IPv4/IPv6) and routing protocols (e.g., OSPF, BGP) operate at this layer to make forwarding decisions. The traceroute command is a common tool that leverages Layer 3 TTL (Time-to-Live) fields to map the path packets take.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Layer 4 - Transport
Why it's wrong here
Layer 4 manages end-to-end delivery, segmentation, and port-based multiplexing between hosts; it does not determine the hop-by-hop path packets traverse. It is tempting because transport protocols reveal connectivity and port failures, making Layer 4 correct when troubleshooting sessions rather than routing paths.
- ✓
Layer 3 - Network
Why this is correct
Layer 3 handles logical addressing and routing, with routers forwarding packets hop-by-hop using IP addresses and routing tables. This satisfies the engineer's need to verify the packet path, since traceroute relies on Layer 3 forwarding decisions rather than Layer 2 switching.
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Layer 1 - Physical
Why it's wrong here
Layer 1 governs transmission of raw bits across the physical medium, so it cannot forward or route packets between networks. It is tempting because cabling and interface faults genuinely cause connectivity loss, making Layer 1 the right focus when diagnosing link, signal or duplex problems rather than path verification.
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Layer 2 - Data Link
Why it's wrong here
Layer 2 forwards frames within a single broadcast domain using MAC addresses, so it cannot route packets between networks or select a path to a remote destination IP. It is tempting because switching is often confused with routing; Layer 2 would be correct for local frame delivery within one subnet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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