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Key Concepts of Ethernet LAN Switching

May 13, 2025

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Jeremy's IT Lab - CCNA Course Lecture Notes

Introduction

  • This course is designed to prepare students for the CCNA exam.
  • The current lecture is the sixth in the series, continuing with Ethernet LAN switching.
  • Emphasis on understanding traffic within a LAN (Local Area Network).

Ethernet Frames and Headers

  • The Ethernet header consists of destination, source, and type fields.
  • The preamble and SFD (Start Frame Delimiter) are not always considered part of the Ethernet header.
  • Total Ethernet header + trailer size: 18 bytes.
  • Minimum Ethernet frame size is 64 bytes (46 bytes payload + 18 bytes header).
  • If payload < 46 bytes, padding (zeroes) is added.

MAC and IP Addresses in LAN

  • Realistic MAC addresses used, with OUI indicating the device manufacturer.
  • A network's IP address (e.g., 192.168.1.0/24) and specific device IPs, e.g., PC1's IP: 192.168.1.1.

Address Resolution Protocol (ARP)

  • Used to discover MAC address of a known IP address.
  • ARP process: ARP request (broadcast) and ARP reply (unicast).
  • ARP request is sent to all devices; ARP reply is sent back to the requester.

Switch Operations and MAC Address Table

  • Switches learn MAC addresses dynamically and enter them into their MAC address table.
  • Known vs. unknown unicast frames and broadcast frames.
  • Commands to view and manage MAC address table:
    • show mac address-table
    • clear mac address-table dynamic

Network Utilities

  • Ping: Tests reachability between two hosts using ICMP echo request/reply.
  • Wireshark: Used for packet captures and analysis.

GNS3 and Packet Tracer

  • GNS3 runs actual Cisco IOS software, requires Cisco IOS purchase.
  • Packet Tracer is free and used for practice labs in this course.

Quiz Questions Recap

  1. Explanation of padding bytes in Ethernet frame.
  2. ARP request is broadcast to all hosts.
  3. Fields in show mac address-table: VLAN, MAC address, type, and ports.
  4. Frames sent to all interfaces by a switch:
    • Broadcast and unknown unicast frames.
  5. Command to clear MAC address table entries on a specific interface.

Additional Resources

  • Anki flashcards and Packet Tracer practice labs available for further study.
  • View supplementary materials to aid in learning.

This lecture covered critical aspects of Ethernet LAN switching and related protocols, essential for understanding network communications in preparation for the CCNA exam.

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