Switching Networking Theory

Broadcast Domain

Understanding broadcast domains and how VLANs and routers segment networks to control broadcast scope

Broadcast Domain

  • A broadcast domain is a logical division of a network where all devices can reach each other at the data link layer (Layer 2) via broadcast
  • When one device sends a broadcast frame (destination MAC FF:FF:FF:FF:FF:FF), all devices in the same broadcast domain receive it
  • Broadcast domains are bounded by Layer 3 devices (routers) - broadcasts do not cross router interfaces by default
  • Each router interface creates a separate broadcast domain

Key Characteristics

  • Size Impact: Larger broadcast domains = more broadcast traffic = potential performance degradation
  • Security Boundary: Devices in same broadcast domain can communicate directly without routing
  • ARP Scope: ARP requests are broadcast frames, so ARP tables only contain entries for devices in the same broadcast domain
  • DHCP Discovery: DHCP Discover messages are broadcasts, requiring DHCP servers or relay agents in each broadcast domain

Device Behavior in Broadcast Domains

Device Type Broadcast Behavior Creates New Domain?
Hub Floods to all ports No
Switch Floods to all ports in same VLAN No (unless VLANs used)
Router Does not forward broadcasts Yes
Layer 3 Switch Forwards within VLAN, routes between VLANs Yes (between VLANs)

VLAN Impact on Broadcast Domains

  • Each VLAN is a separate broadcast domain - this is the primary purpose of VLANs
  • Switch ports in VLAN 10 cannot receive broadcasts from VLAN 20
  • Default VLAN 1 includes all switch ports initially
  • Inter-VLAN communication requires Layer 3 routing (router or Layer 3 switch)

Practical Examples

Single Broadcast Domain:

  • 24-port switch with all ports in default VLAN
  • All 24 connected devices receive every broadcast
  • One device’s ARP request reaches all 23 other devices

Multiple Broadcast Domains:

  • Same switch configured with VLANs 10, 20, 30
  • Ports 1-8 in VLAN 10, ports 9-16 in VLAN 20, ports 17-24 in VLAN 30
  • Creates 3 separate broadcast domains on single physical switch
  • ARP request from VLAN 10 device only reaches other VLAN 10 devices

Router-Separated Domains:

  • Router with Gi0/0 connected to Switch A, Gi0/1 connected to Switch B
  • Switch A devices cannot receive broadcasts from Switch B devices
  • Each router interface represents different IP subnet (e.g., 192.168.1.0/24 and 192.168.2.0/24)

Vocabulary

  • Broadcast Frame: Layer 2 frame with destination MAC FF:FF:FF:FF:FF:FF
  • Flooding: Switch behavior of forwarding frame out all ports (except source port)
  • ARP Request: Broadcast asking “Who has IP address X.X.X.X?”
  • DHCP Discover: Broadcast from client seeking DHCP server
  • Inter-VLAN Routing: Layer 3 process enabling communication between VLANs

Notes

  • Collision domains and broadcast domains are different concepts - modern switches create separate collision domain per port but single broadcast domain per VLAN
  • Broadcast storms can occur when switching loops exist without Spanning Tree Protocol (STP)
  • Use show vlan brief to see VLAN assignments and identify broadcast domain boundaries
  • Routers will forward directed broadcasts if configured with ip directed-broadcast (disabled by default for security)
  • Wireless access points typically bridge wireless clients into same broadcast domain as wired network
  • Best practice: Keep broadcast domains reasonably sized (typically under 250-500 devices) to minimize broadcast overhead