IEEE 802.1Q VLAN tagging is one of the most fundamental technologies used in enterprise networking. It enables logical segmentation of networks while allowing multiple VLANs to share the same physical infrastructure through trunk links.
This master-level guide explains the VLAN tagging diagram from an enterprise architect perspective, including frame processing, packet tagging, switch behavior, security risks, and real-world deployment models.
1. High-Level Architecture Overview
The diagram shows two switches connected by a trunk link. Multiple VLANs travel across the trunk while end devices communicate using standard Ethernet frames.
- Access ports connect endpoints.
- Trunk ports carry tagged traffic.
- Switches add and remove VLAN tags automatically.
2. Access Ports (Untagged Traffic Flow)
Access ports connect devices such as PCs, printers, and servers. Frames arriving from endpoints do not include VLAN tags.
Internal Switch Behavior
- Switch assigns VLAN ID based on port configuration.
- Frame stored in VLAN-specific forwarding table.
- Broadcast traffic limited to that VLAN only.
3. Trunk Ports and Tagged Frames
Trunk ports allow multiple VLANs to share a single physical link between switches. The switch inserts an 802.1Q tag into Ethernet frames before transmission.
802.1Q Header Structure
- TPID – Tag identifier.
- PCP – Priority bits for QoS.
- DEI – Drop eligibility.
- VLAN ID – Identifies VLAN membership.
4. Packet Flow Explained (Step-by-Step)
- Device sends untagged Ethernet frame.
- Switch assigns VLAN internally.
- Frame tagged when leaving trunk port.
- Receiving switch reads VLAN ID.
- Frame forwarded to correct VLAN access port.
- Tag removed before delivery to endpoint.
5. Switch ASIC Processing (Real Internal Logic)
Enterprise switches use ASICs to process frames at hardware speed. When a tagged frame arrives:
- ASIC reads VLAN ID.
- Looks up MAC address in VLAN-specific CAM table.
- Applies forwarding rules.
6. Enterprise VLAN Design Strategy
- Separate user VLANs by department.
- Create voice VLAN for IP phones.
- Isolate IoT devices.
- Separate guest network.
- Use management VLAN for network devices.
7. Inter-VLAN Routing (Layer 3 Switching)
VLANs are isolated by default. Layer 3 routing is required for communication between VLANs.
- SVI (Switched Virtual Interfaces).
- Layer 3 switches perform routing.
- ACLs control cross-VLAN access.
8. VLAN Security Threats (Architect Perspective)
VLAN Hopping Attack
- Double-tagging exploits trunk misconfiguration.
- Allows access to unintended VLAN.
Native VLAN Attacks
- Default VLAN usage can cause leakage.
9. Security Best Practices
- Change native VLAN from default.
- Disable unused trunk ports.
- Restrict allowed VLAN list on trunks.
- Use Private VLANs where required.
10. Architect-Level Insight
VLAN tagging is foundational but modern enterprise networks often combine VLAN segmentation with Zero Trust networking, micro-segmentation, and software-defined networking technologies.
Conclusion
Understanding VLAN tagging at packet level enables better design decisions. Proper trunk configuration, security hardening, and inter-VLAN routing policies are essential for scalable and secure enterprise networks.
