Network Models and Encapsulation

Learning objectives

• Explain the purpose and scope of Network Models and Encapsulation.
• Identify important protocols, roles, fields, and terminology.
• Apply the concept to a practical network scenario.
• Use IOS XE commands to verify operational behavior.

1. Core concepts

OSI has seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. TCP/IP commonly groups these functions into Link, Internet, Transport, and Application. These are conceptual models; real protocols do not always fit a single layer perfectly.

2. How it works

Common PDUs are bits at Layer 1, frames at Layer 2, packets at Layer 3, and segments (TCP) or datagrams (UDP) at Layer 4. As data moves down the stack, layers add headers and sometimes trailers. At the receiver, each layer processes and removes its own information.

Visual walkthrough

3. Worked example

When a router forwards an IP packet, it removes the incoming Layer 2 frame and creates a new frame for the outgoing link. Source and destination MAC addresses change at each routed hop, while end-to-end IP addresses normally remain unchanged unless NAT is used.

IOS XE commands and verification

Syntax and feature availability can vary by device family and software release. Confirm platform support before applying configuration.

show interfaces
show ip interface brief
show ip route
ping <destination>
traceroute <destination>

Common mistakes and troubleshooting

• Check physical and interface state before changing higher-layer settings.
• Verify both ends agree on addressing, VLAN, encapsulation, and protocol parameters.
• Use operational show commands; configured text alone does not prove a feature is working.
• Change one variable at a time and verify the result.

Knowledge check

Choose one answer per question. Submit to see why the correct choice is right and why every other option is wrong. These are original practice questions, not Cisco exam questions.