Collision domain: where frames can collide (only relevant for half-duplex). Broadcast domain: where broadcasts (ff:ff:ff:ff:ff:ff) are forwarded.
Collision domain
What is it? A network segment where two devices can transmit at the same time and cause collisions.
- OSI layer: L1/L2
- Relevant technology: Ethernet (CSMA/CD – historical)
How is it divided?
- Hub → one large collision domain
- Switch → one collision domain per port
- Router → stops collision domains (one per interface as well)
Important (exam!)
- In full-duplex Ethernet there are no collisions → collision domains are effectively irrelevant in modern networks.
- CCNA questions often use hubs to test the concept.
Broadcast domain
What is it? An area where Layer 2 broadcasts are forwarded to all devices.
- OSI layer: L2 (bounded by L3 boundaries)
- Examples of broadcasts: ARP request, DHCP Discover
How is it divided?
- Switch (no VLANs) → one broadcast domain
- VLAN → each VLAN = one broadcast domain
- Router / L3 switch → stops broadcasts
Important (exam!) VLANs divide broadcast domains, not collision domains. Inter-VLAN communication requires routing.
Quick comparison
| Device / technology | Collision domain | Broadcast domain |
|---|---|---|
| Hub | 1 | 1 |
| Switch (L2) | 1 per port | 1 (without VLANs) |
| Switch with VLANs | 1 per port | 1 per VLAN |
| Router / L3 SW | 1 per interface | 1 per interface |
Typical exam traps ⚠️
- “A switch does not stop broadcasts” → correct (unless VLAN/L3).
- “VLAN = collision domain” → wrong (VLAN = broadcast domain).
- “Collision domains still matter in full-duplex” → wrong.
Quick memory rule (30 sec)
- Collision = who can crash into each other? → hub bad, switch good
- Broadcast = who hears the shout? → VLAN/router set the boundaries
- A switch splits collision domains; a router/VLAN splits broadcast domains
The Cisco-book explanation
Basic rule (from the books): every Layer 2 interface = one collision domain. A VLAN has no influence on collision domains.
Concrete scenario
PC ──> Switchport (VLAN 10) ──> Router interface (VLAN 10)Cisco model: PC → switch port = 1 collision domain; switch port → router interface = 1 collision domain.
➡️ The router interface is its own collision domain. ➡️ The switch port is its own collision domain.
❗ They are not merged, even when they are in the same VLAN, the same IP subnet and the same broadcast domain.
The important distinction (exam!)
| Concept | Controlled by |
|---|---|
| Collision domain | Interface / port |
| Broadcast domain | VLAN / router boundary |
VLAN = broadcast domain – not a collision domain.
Correct exam sentence: “Even if a switch port and a router interface are in the same VLAN, they still represent separate collision domains.” That is fully correct per Cisco Press ✅
Typical trap: ❌ “Same VLAN → same collision domain” (wrong). ✔️ “Same VLAN → same broadcast domain” (correct).
Ultra-short summary
Switch ↔ router in the same VLAN: same broadcast domain, but separate collision domains.