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Ethernet LAN Switching Overview

Jun 22, 2025

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Overview

This lecture covers Ethernet LAN switching, focusing on how switches forward frames within a LAN using MAC addresses, Ethernet frame structure, and the MAC address table learning process.

OSI Model Review

  • The Physical Layer (Layer 1) defines the physical characteristics for data transfer, such as voltage, maximum cable length, and connectors.
  • The Data Link Layer (Layer 2) provides node-to-node data transfer and uses Layer 2 (MAC) addresses.
  • Switches operate at Layer 2, using MAC addresses to forward data.

LANs and Switches

  • A LAN is a network within a small area (e.g., office, home) and is not separated by switches but by routers.
  • Devices within the same LAN are connected via switches; different LANs are separated via routers.

Ethernet Frame Structure

  • An Ethernet frame consists of a header and trailer encapsulating the data.
  • Header fields:
    • Preamble (7 bytes): Used for receiver clock synchronization.
    • Start Frame Delimiter (SFD, 1 byte): Indicates the start of the frame.
    • Destination MAC address (6 bytes): Target device’s MAC address.
    • Source MAC address (6 bytes): Sender’s MAC address.
    • Type/Length (2 bytes): Indicates packet type (if ≥1536) or length (if ≤1500 bytes).
  • Trailer field:
    • Frame Check Sequence (FCS, 4 bytes): Detects errors using a cyclic redundancy check (CRC).

MAC Addresses and Hexadecimal

  • A MAC address is a 6-byte (48-bit) globally unique physical address assigned at manufacture.
  • The first 3 bytes (OUI) identify the manufacturer; last 3 bytes uniquely identify the device.
  • MAC addresses are displayed in hexadecimal, which uses 0-9 and A-F as digits (16 possible values per digit).

Switch Operation and MAC Address Table

  • Switches learn MAC addresses dynamically by examining the source address of incoming frames.
  • When a switch receives a frame with an unknown destination MAC (unknown unicast), it floods the frame out all ports except the incoming one.
  • When the destination MAC is known, the switch forwards the frame only to the relevant port.
  • Dynamic MAC table entries age out after 5 minutes of inactivity on Cisco switches.

Types of Ethernet Frames

  • Unicast: Frame is destined for a single recipient.
  • Unknown unicast: Destination MAC not in switch table; frame is flooded.
  • Known unicast: Destination MAC is known; frame is forwarded directly.

Key Terms & Definitions

  • MAC address — 48-bit (6-byte) physical address assigned to network devices.
  • OUI (Organizationally Unique Identifier) — First 3 bytes of a MAC address that identifies the manufacturer.
  • Unicast Frame — Frame sent to a single target MAC address.
  • Unknown Unicast — Frame sent to a MAC address not in the switch table; gets flooded.
  • Preamble — 7-byte field for receiver clock synchronization.
  • Frame Check Sequence (FCS) — 4-byte CRC field for error checking.

Action Items / Next Steps

  • Download and review Anki flashcards for this lesson.
  • Prepare for a Packet Tracer lab (to be completed after Ethernet LAN Switching Part 2).
  • Review this material before moving to the next lesson on inter-LAN communication.

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