What happens to broadcast traffic with and without VLANs in a multi-switch network?

Without VLANs (Virtual Local Area Networks), a broadcast from any device floods to ALL devices on ALL switches. With VLANs, broadcasts are confined to only devices in the same VLAN.

VLAN segmentation confines broadcasts per VLAN. — Luca Ghio, CC BY-SA 4.0, via Wikimedia Commons.

Without VLANs:

• A Faculty PC sends a broadcast
• All switches forward it out every port
• Every device — Faculty, Student, and Guest — receives it, even on remote switches
• All devices share one big broadcast domain (all on the same 172.17.40.0/24 subnet)

With VLANs:

• A Faculty PC in VLAN 10 sends a broadcast
• Switches only forward it out ports in VLAN 10 and over trunks
• Only the other Faculty PC (also VLAN 10) receives the broadcast
• Every device in a different VLAN (Student, Guest) is unaffected
• Each VLAN has its own subnet (VLAN 10 = 172.17.10.0/24, VLAN 20 = 172.17.20.0/24)

Critical concept: Without a Layer 3 device (router), devices in different VLANs cannot communicate at all — even if they're on the same physical switch. VLANs create complete Layer 2 isolation.

Go deeper:

• Virtual LAN (Wikipedia) — a VLAN is a separate broadcast domain, so broadcasts stay within the VLAN across trunked switches.