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Unit 10: Network Traffic TypesLesson 10.2 (2 of 2 in this unit)62 of 84 in the Network Fundamentals course

Broadcast domains

How far a broadcast travels, and what stops it.

Beginner · 6 min read

Broadcast domain is the group of devices that receive one another's Layer 2 broadcast frames. Switches forward a broadcast out of every port in the same VLAN, but routers do not forward it to other interfaces, so a broadcast domain usually matches one VLAN and one IP subnet.

In simple terms: It is the group of devices that hear a message sent to “everyone”. A router or a VLAN boundary acts as a wall that the message does not cross.

What a broadcast domain is

A broadcast frame (destination FF:FF:FF:FF:FF:FF) is meant for every device on the local network. ARP requests and DHCP Discover messages are everyday examples. A broadcast domain is the set of devices that receive each other's broadcasts. In IP terms, it is usually one subnet (one VLAN).

Switches pass broadcasts; routers stop them

PC ASW1SW2PC BRouterPC Cother subnet
  1. 1. Switches flood it. PC A's ARP request is flooded out of every port of SW1 and SW2, including the link between them.
  2. 2. Every device in the domain receives it. PC B receives it, and so does the router's interface on this subnet.
  3. 3. The router doesn't forward it. Routers don't forward Layer 2 broadcasts between interfaces, so PC C never sees it. The router marks the edge of the broadcast domain.

VLANs split broadcast domains too

A switch with two VLANs (virtual LANs) behaves like two separate switches: a VLAN 10 broadcast reaches only VLAN 10 ports. So each VLAN is its own broadcast domain, and you need a router (or a Layer 3 switch) to move traffic between them. The VLAN lesson shows this as an animation.

Learn more: VLANs

Why size matters

Every device in a broadcast domain must receive and process every broadcast. With a few dozen devices, that is no problem. With thousands, broadcasts use up bandwidth and CPU time on every host, and a single misbehaving device (or a switching loop) affects every device in the domain. That is why networks are divided into VLANs and subnets of manageable size.

Counting broadcast domains

DeviceCollision domainsBroadcast domains
HubOne for all portsOne for all ports
Switch (one VLAN)One per portOne for all ports
Switch (several VLANs)One per portOne per VLAN
RouterOne per interfaceOne per interface

Check yourself

Predict · scenario 1

A router has two LAN interfaces. Interface 1 connects to two switches linked together (no VLANs); interface 2 connects to a single switch. How many broadcast domains are there?

Predict · scenario 2

You configure a switch with ports in VLANs 10, 20 and 30. A PC in VLAN 10 sends an ARP broadcast. How many broadcast domains does the switch provide?