Networking Concepts: VLANs and LANs

Study Notes

LAN Overview

A LAN (Local Area Network) is a single broadcast domain comprising all devices that receive broadcast frames.
A broadcast domain refers to the group of devices responding to a broadcast frame with a specified destination MAC address (FFFF.FFFF.FFFF).

VLAN Introduction

VLAN (Virtual Local Area Network) logically segments end hosts at Layer 2 of the OSI model, improving network management and performance.
Reduces unnecessary broadcast traffic, enhancing overall network performance.
Provides security by limiting access and allowing tailored security policies through routers and firewalls.

VLAN Configuration Basics

VLANs are set up on switches per interface, with VLAN 1 and VLANs 1002-1005 existing by default and non-deletable.
Access ports belong to a single VLAN, usually connecting to end devices, while trunk ports can support multiple VLANs.

Trunk Port Functionality

In small networks, individual interfaces can connect switches and routers for each VLAN, but this becomes impractical as VLAN numbers increase.
Trunk ports consolidate traffic from multiple VLANs over a single interface, optimizing interface usage.
Frames sent over trunk links are tagged for identification by receiving switches, distinguishing between access ports (untagged) and trunk ports (tagged).

VLAN Tagging Protocols

Two primary trunking protocols exist:
- ISL (Inter-Switch Link): An older, Cisco proprietary protocol.
- IEEE 802.1Q: The industry standard protocol commonly referred to as dot1q.

802.1Q Tag Structure

The 802.1Q VLAN tag is 4 bytes (32 bits) in length, comprising two fields:
- Tag Protocol Identifier (TPID): 16 bits indicating the frame is 802.1Q tagged with a fixed value of 0x8100.
- Tag Control Information (TCI): 16 bits broken into three sub-fields.

TCI Sub-fields

PCP (Priority Code Point): 3 bits used for Class of Service (CoS) to prioritize important traffic.
DEI (Drop Eligible Indicator): 1 bit indicating frames that may be dropped during congestion.
VID (VLAN ID): 12 bits identifying the VLAN, with a range of 1-4094 (0 and 4095 are reserved).

VLAN Ranges

VLANs are categorized into:
- Normal VLANs: 1-1005.
- Extended VLANs: 1006-4094.

Native VLAN Concept

The native VLAN is set to VLAN 1 by default on all trunk ports but can be changed manually.
Untagged frames received on trunk ports are assumed to belong to the native VLAN, making configuration consistency crucial.

Trunk Configuration Summary

Proper configuration of trunk ports is essential for seamless VLAN communication and network efficiency.

Description

Explore the fundamentals of VLANs and LANs with this quiz. Learn about broadcast domains, their impact on network performance, and how many broadcast domains can be identified in a given network. Test your understanding of these critical networking concepts.