Cheat Sheet: Fundamentals of Ethernet LANs (Chapter 2)

Core Concepts (Key Points)

• LAN Types:
  • Ethernet LANs: Use cables (copper or fiber) for connectivity.
  • Wireless LANs: Use radio waves, defined by IEEE 802.11.
• Ethernet LAN Standards:
  • Defined by IEEE 802.3 for both physical and data-link layers.
  • Physical layer variations include speed (e.g., 10 Mbps to 400 Gbps) and media (UTP, fiber).
• Cabling Types:
  • Copper (UTP): Inexpensive, limited to 100m.
  • Multimode Fiber: Supports longer distances, cost-effective for medium ranges.
  • Single-Mode Fiber: Longest distances (up to 40 km), higher cost.

Important Definitions

• MAC Address: A unique 6-byte hardware address assigned to NICs.
• Broadcast Address: FFFF.FFFF.FFFF, used to send frames to all devices.
• Multicast Address: Delivers frames to a group of devices on a LAN.
• Unicast Address: Identifies a single LAN device.

Important Examples

• SOHO LAN Topology (Figure 2-1):
  • Ethernet switch connects PCs, printers, and routers in a small office/home office setup.
• Enterprise LAN (Figure 2-3):
  • Includes switches on multiple floors connected by distribution switches.

Key Commands

(Not applicable for this chapter; focus is on concepts.)

Common Terms

• RJ-45: Standard connector for Ethernet cables.
• Ethernet Frame: Includes source/destination MAC addresses, data, and FCS.
• Full Duplex: Allows simultaneous sending and receiving.
• Half Duplex: Requires devices to alternate sending and receiving.

Practical Tips

• Choosing Cables:
  • Use straight-through cables for different device types (e.g., PC to switch).
  • Use crossover cables for same device types (e.g., switch to switch).
• Auto-MDIX: Automatically corrects cable pinout mismatches in modern devices.

Visual Representation

• Ethernet Frame Format:

Preamble | SFD | Dest MAC | Src MAC | Type | Data | FCS
   7      |  1  |    6     |    6    |  2   | 46–1500 | 4

• SOHO LAN Example:

[PC] ---- [Switch] ---- [Router] ---- [Internet]

• Fiber Cable Layers (Figure 2-14):
  • Core, Cladding, Buffer, Strengthener, Outer Jacket.

Exam-Focused Points

• Cable Types:
  • Straight-Through: Connects devices like PC-to-switch.
  • Crossover: Connects devices like switch-to-switch.
  • Fiber Standards: E.g., 10GBASE-LR supports up to 10 km.
• Ethernet Addressing:
  • OUI: First 3 bytes of MAC address identify the manufacturer.
  • MAC addresses must be unique to avoid conflicts.
• Collision Domain vs. Broadcast Domain:
  • Collision Domain: Network area where collisions can occur (e.g., hubs).
  • Broadcast Domain: Devices receiving the same broadcast frame.

Quick Troubleshooting Guide

• Connectivity Issues:
  • Check physical connections and cable types.
  • Ensure devices support the same Ethernet standards (e.g., 100BASE-T).
• Performance Issues:
  • Inspect for EMI or incorrect duplex settings.

Recommended Further Reading

• IEEE 802.3 Standards: For detailed Ethernet specifications.
• Ethernet Alliance Website: Up-to-date roadmap of Ethernet standards.
• CCNA 200-301 Official Cert Guide, Volume 1 , Second Edition Chapter 3: Deeper understanding of IP and routing.