Two ideas explain a lot about how Ethernet networks behave as they grow: the collision domain and the broadcast domain. A collision domain is the set of devices whose transmissions can collide with each other on a shared medium. A broadcast domain is the set of devices that all receive a Layer 2 broadcast frame (one sent to the destination MAC address ff:ff:ff:ff:ff:ff) when any one of them sends it. The JNCIA-Junos exam expects you to know which devices split which domain, and why that matters for performance.
Collisions come from the early days of Ethernet, when every station shared one cable or was connected through a hub. A hub is a Layer 1 repeater: whatever arrives on one port is copied out every other port, so only one device can transmit at a time. If two send at once, the signals collide, both stop, wait a random time and try again. That access method is CSMA/CD (carrier sense multiple access with collision detection), and it only applies to half-duplex links. Every port on a hub is in the same collision domain, so adding devices means more collisions and less usable bandwidth.
A switch fixes this. Each switch port is its own collision domain, because the switch buffers frames and forwards them only where they need to go. When a port runs full duplex, which is normal today, the device can send and receive at the same time and collisions cannot happen at all. So a 24-port switch creates 24 collision domains.
A switch does not, however, stop broadcasts. By default it floods a broadcast frame out every port in the same VLAN (virtual LAN) except the one it came in on. All those ports form one broadcast domain. Broadcasts are necessary, because ARP requests and DHCP discovery use them, but in a very large flat network they consume bandwidth and CPU on every host. VLANs let one switch create several separate broadcast domains: each VLAN is its own broadcast domain.
A router divides broadcast domains. It does not forward Layer 2 broadcasts from one interface to another, so each router interface (or each logical unit, on Junos) sits in a different broadcast domain and a different IP subnet. Traffic between broadcast domains must be routed, either by a router or by a Layer 3 switch using an integrated routing and bridging (IRB) interface.
A handy summary: hubs split nothing, switches split collision domains, VLANs and routers split broadcast domains. When you count domains in an exam diagram, count each switch port and router interface as a separate collision domain (a hub and everything on it is one), and count each router interface or VLAN as a separate broadcast domain.
Key terms
- Collision domain
- A network segment where simultaneous transmissions can collide; each switch or router port is its own collision domain.
- Broadcast domain
- The set of devices that receive a Layer 2 broadcast from any member; bounded by routers and by VLANs.
- CSMA/CD
- Carrier sense multiple access with collision detection, the half-duplex Ethernet method of listening, detecting collisions and backing off.
- Full duplex
- A link mode where a device can send and receive at the same time, which removes collisions entirely.
- VLAN
- Virtual LAN: a logical Layer 2 segment on a switch; each VLAN is a separate broadcast domain.
An office has 40 PCs on two 24-port EX switches in one VLAN, with an SRX firewall as the gateway. There are about 48 collision domains (one per used switch port) but only one broadcast domain on the LAN side. When the team splits users into a Staff VLAN and a Guest VLAN, the switch now has two broadcast domains, and the SRX (or an IRB interface) must route between them.
Check yourself
How many broadcast domains does a single switch with all ports in the default VLAN create?
One. A switch floods broadcasts out every port in the same VLAN, so without extra VLANs the whole switch is one broadcast domain.
Why can collisions not occur on a full-duplex switch port?
Because the device and the switch port each have a dedicated path to send and receive at the same time, so there is no shared medium for signals to collide on.
What device or feature do you need to separate broadcast domains?
A router (or Layer 3 interface such as an IRB) or VLANs; each router interface and each VLAN is its own broadcast domain.