IEEE 802.1Q VLAN Standard

IEEE 802.1Q defines Virtual Local Area Networks (VLANs), a method for logically segmenting a single physical network into multiple isolated broadcast domains. A VLAN tag (16-bit field) inserted into the Ethernet frame header identifies which VLAN a packet belongs to. In AV networks, VLANs separate Dante audio, video-over-IP, control traffic, and management from general IT data, enabling independent QoS policies, security, and bandwidth guarantees per stream type.

802.1Q VLAN tagging is the foundation of modern managed switches and is essential for deterministic AV transport (Dante, AVB, AES67) over shared Ethernet infrastructure. Unlike physical segmentation (separate cables), VLANs reuse the same switch ports while logically isolating traffic.

Key Specifications

VLAN Tag: 4-byte header inserted between source MAC and EtherType; includes 12-bit VLAN ID (VID), allowing 4,094 user VLANs (0 and 4095 reserved). Priority bits enable QoS (802.1p class-of-service).

Tagging Methods:

• Tagged: VLAN tag present in frame; used on trunk ports between switches.
• Untagged (Native VLAN): No tag; frames default to native VLAN on that port. Common for endpoints.

Port Types:

• Access port: Accepts untagged frames, assigns a single VLAN.
• Trunk port: Accepts tagged and untagged; carries multiple VLANs between switches.

Spanning Tree: 802.1D/802.1w (RSTP) prevents loops in redundant topologies; often paired with VLAN configuration for split-horizon and failover.

Multiple Spanning Trees (MSTP): 802.1s allows separate spanning-tree instances per VLAN group, optimizing failover in large networks.

Practical Application for AV

In a Dante network, separate VLANs for audio (e.g., VLAN 100), video (VLAN 101), and management (VLAN 10) allow:

• Independent QoS: Audio VLAN prioritized (PCP=5), best-effort IT traffic deprioritized.
• Bandwidth isolation: If IT users saturate VLAN 10, Dante streams on VLAN 100 are unaffected.
• Security: Audio traffic doesn't cross to IT; reduces access to sensitive AV infrastructure.
• Scalability: Add new VLANs without rewiring; configure in switch and endpoints.

Typical setup: Dante endpoints connected to Dante-capable switches (e.g., Cisco, Dante-Via, Audinate) with VLAN 100 trunk to audio system, VLAN 10 for Dante Controller management. See vlan-configuration-for-av for detailed examples.

In large theaters or broadcast facilities, redundant rings use VLAN trunking on multiple uplinks with MSTP to ensure failover without loops. AES67 streams may use separate VLANs per talker to simplify monitoring and reservation.

Common Pitfalls

1. Untagged chaos: Endpoints not configured for VLAN 100; frames default to native VLAN (often VLAN 1), breaking multicast discovery. Always verify endpoint VLAN membership.
2. Native VLAN mismatch: Switch trunk port set to native VLAN 1; untagged Dante frames misroute. Align native VLAN on both sides.
3. Spanning-tree loops: Adding redundant links without RSTP enabled; broadcast storms crash audio. Enable RSTP (fast convergence, loop prevention).
4. QoS not applied to VLAN: VLAN created but PCP priority not enforced; IT traffic starves Dante. Verify switch QoS queue discipline per VLAN.