Breaking Broadcast Domain Through VLAN and Router – How to

Theory

By convention, a collision domain is a contiguous wired and / or wireless half-duplex segment (typically using CSMA/CD), which is a subset of the subnet or vlan's broadcast domain.

• Collision domains are invariably a half-duplex segment, or a series of connected half-duplex segments.
  • Ethernet hubs / Wifi Access Points (within the same RF-channel width) start or continue a collision domain
  • Ethernet switches, routers, firewalls all form a boundary on a collision domain.
• Broadcast domains could have multiple half-duplex segments inside them.

How Ethernet switches control Collision Domains

Ethernet switches control collision domains because they can join half-duplex and full-duplex links within the same broadcast domain; Ethernet switches buffer frames to assist the transition between half-duplex and full-duplex links.

Example:

This shows a strange network which illustrates how collision domains are a contained within a broadcast domain, and how devices form boundaries for each one.

• Yellow areas are the span of the broadcast domain
• Grey area are the span of the collision domains
• Firewalls and routers are assumed in routed mode (i.e. not bridged mode)

Each VLAN creates its own broadcast domain in 1 or more physical switches. If you have a switch taken out of the box it tends to put all ports in vlan 1 so for say a 24 ports the broadcast domain includes all 24 ports. If you were to create a vlan 2 and configure half the ports to be members of vlan 2 you then have 2 broadcast domains each of 12 ports. if you create a vlan 3 and put half the ports that are still in vlan 1 in it you would end up with 3 broadcast domains 2 with 6 ports and another 1 with 12 ports.