Cisco CCNA Preparation · lesson 3 of 36 · 25 min
Why the OSI Model Still Matters
The OSI (Open Systems Interconnection) model is a conceptual framework that standardises how network functions are divided into seven layers. It was created by ISO in 1984 to allow different vendors’ equipment to interoperate.
In practice, TCP/IP (a 4-layer model) is what the internet actually uses. But the OSI model remains essential for the CCNA exam because it provides a precise vocabulary for troubleshooting, protocol classification, and understanding where security controls operate.
Exam tip
The CCNA exam tests OSI heavily in troubleshooting scenarios. “At which layer does X operate?” is a common question format. Know every layer’s PDU name, protocols, and devices.
The Seven Layers
Encapsulation and De-encapsulation
Encapsulation is the process of adding layer-specific headers (and sometimes trailers) as data moves down the OSI stack from Application to Physical. Each layer wraps the data from the layer above with its own control information.
De-encapsulation is the reverse — stripping headers as data moves up the stack at the receiving end.
Encapsulation — sender side (Application → Physical)
Key insight
Routers de-encapsulate to Layer 3 (read the IP header to make routing decisions), then re-encapsulate with a new Layer 2 header for the next hop. Switches only de-encapsulate to Layer 2 (read the MAC address). This is why routers can connect different network types but switches cannot.
Layer Functions in Detail
TCP/IP Model vs OSI Model
Key Terms
Key Terms
Knowledge Check — 10 Questions
OSI Model Deep Dive
At which OSI layer does a router primarily operate?
- Layer 1
- Layer 2
- Layer 3
- Layer 4
Answer: Layer 3
Routers operate at Layer 3 (Network). They read IP headers to make forwarding decisions and connect different networks.
What is the PDU name at OSI Layer 2?
- Packet
- Segment
- Frame
- Bit
Answer: Frame
Layer 2 (Data Link) PDU is a Frame. It includes source and destination MAC addresses plus an FCS trailer for error detection.
Which layer adds source and destination port numbers?
- Layer 2
- Layer 3
- Layer 4
- Layer 7
Answer: Layer 4
Layer 4 (Transport) adds port numbers in the TCP or UDP header to identify the application on each end.
TLS encryption operates at which OSI layer?
- Layer 3
- Layer 4
- Layer 5
- Layer 6
Answer: Layer 6
TLS operates at Layer 6 (Presentation), which handles encryption, compression, and data formatting.
When a router receives a frame, it de-encapsulates to which layer to make a forwarding decision?
- Layer 1
- Layer 2
- Layer 3
- Layer 4
Answer: Layer 3
Routers strip the Layer 2 frame header and read the Layer 3 IP header to determine the destination network and next hop.
Which OSI layer is responsible for establishing, managing, and terminating sessions between applications?
- Layer 4
- Layer 5
- Layer 6
- Layer 7
Answer: Layer 5
Layer 5 (Session) manages dialogues between applications — establishing, maintaining, and terminating sessions.
In the TCP/IP model, which layer combines OSI Layers 5, 6, and 7?
- Transport
- Internet
- Application
- Network Access
Answer: Application
The TCP/IP Application layer combines OSI Session (5), Presentation (6), and Application (7) into a single layer.
What does a switch primarily use to forward frames?
- IP addresses (Layer 3)
- MAC addresses (Layer 2)
- Port numbers (Layer 4)
- Domain names (Layer 7)
Answer: MAC addresses (Layer 2)
Switches operate at Layer 2 and forward frames based on destination MAC addresses in their MAC address table.
The process of adding layer headers as data moves down the OSI stack is called _____.
Answer: encapsulation
Encapsulation wraps data with layer-specific headers at each layer. The reverse process (stripping headers at the receiver) is de-encapsulation.
OSPF routing protocol updates operate at which OSI layer?
- Layer 2
- Layer 3
- Layer 4
- Layer 7
Answer: Layer 3
OSPF is a Layer 3 routing protocol. It uses IP protocol number 89 and operates directly over IP without TCP or UDP.
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