2.2 Enterprise Switching: VLANs, 802.1Q Trunks & Spanning Tree Protocol

🎯 Key CompTIA Network+ Exam Takeaways

• VLANs partition physical switches into isolated Layer 2 broadcast domains, requiring a Layer 3 router for inter-VLAN communications.
• IEEE 802.1Q inserts a 4-byte tag into Ethernet frames; native VLAN traffic is transmitted untagged by default.
• STP (802.1D/802.1w) elects the switch with the lowest Bridge ID (Priority + MAC) as Root Bridge to prevent bridging loops.

Virtual Local Area Networks (VLANs) segment physical switches into logical broadcast domains, improving security and network efficiency. To carry multiple VLANs across an inter-switch link, 802.1Q trunking inserts a 4-byte header containing a 12-bit VLAN identifier.

Spanning Tree Protocol (STP) prevents devastating broadcast storms by blocking redundant physical paths while keeping them available as backups. Switches exchange Bridge Protocol Data Units (BPDUs) to elect a Root Bridge based on the lowest Bridge ID.

Rapid Spanning Tree Protocol (RSTP 802.1w) dramatically accelerates convergence from 30-50 seconds down to sub-second intervals using active edge port synchronization.

⚠️ Common Pearson VUE Exam Traps

• Allowing mismatched native VLANs on opposite ends of an 802.1Q trunk, which causes cross-VLAN traffic leakage.
• Assuming the switch with the highest MAC or priority wins the STP election; STP always prioritizes the lowest numerical value.