Fundamentals of Switches and Ethernet Frame Structure

Switch: data exchange

Data Link Layer: establishment, maintenance and termination of data links, data encapsulation (frames), frame transmission, frame synchronization, and frame error detection.

MAC address: composed of 48 bits, the first 24 bits are the manufacturer identifier and the last 24 bits are the NIC identifier; usually represented in hexadecimal.

Unicast address: the 8th bit is 0, 1‑to‑1 communication.

Multicast address: the 8th bit is 1, 1‑to‑many communication.

Ethernet II frame format consists of six parts:

Preamble: frame delimiter, 1 byte, total 8 bytes.

Destination address: 6 bytes, destination MAC address.

Source address: 6 bytes, source MAC address.

Type: 2 bytes, indicates the upper‑layer protocol type (e.g., 0x0800 for IP).

Data: 46–1500 bytes.

Frame Check Sequence: 4 bytes, checksum.

Preamble

Destination address

Source address

Type

Data

FCS

Switch categories include major vendors such as Cisco, Huawei, H3C, and TP‑Link.

No.

Series

Function

1

Cisco 2960

Entry‑level switch

2

Cisco 3560

Enterprise‑level switch, directly connects client hosts, entry‑level switch (with routing capability)

3

Cisco 4500

Modular switch, expandable modules to assist enterprise service deployment

4

Cisco 6500

High‑end switch for large enterprise networks

Switch forwarding principle:

1. Host A sends data to Host B.

2. Switch AA receives the data.