QinQ (802.1ad)
- Overview
QinQ (IEEE 802.1ad), also known as Stacked VLANsor Double Tagging, is an Ethernet networking standard designed to extend VLAN scalability in service provider (SP) networks. It encapsulates customer VLAN (C-VLAN) frames within an additional service provider VLAN (S-VLAN) tag, enabling hierarchical network segmentation and efficient traffic management across multi-tenant environments.
- Key Functions
- VLAN Expansion: Overcomes the 4,094 VLAN limit of standard 802.1Q by adding a second VLAN tag, effectively supporting up to ~16 million logical segments (4,094 × 4,094).
- Traffic Isolation: Segregates customer traffic using S-VLANs, ensuring overlapping C-VLAN IDs from different clients do not conflict.
- Service Transparency: Preserves customer VLAN configurations end-to-end, allowing enterprises to retain internal VLAN architectures without SP interference.
- Simplified SP Operations: Enables SPs to manage aggregated customer traffic via S-VLANs while delegating C-VLAN control to clients.
- Frame Format and Protocol Details
- Dual Tagging Mechanism:
- Outer Tag (S-Tag): Added by the SP edge device (e.g., PE switch), using a Service VLAN ID (S-VID) and a TPID (Tag Protocol Identifier) value of 0x88A8 (per 802.1ad).
- Inner Tag (C-Tag): Original customer VLAN tag (TPID 0x8100 per 802.1Q) with a Customer VLAN ID (C-VID).
- EtherType: The outer tag's EtherType is set to 0x88A8 to distinguish it from standard 802.1Q frames.
- Frame Structure:
[Destination MAC][Source MAC][0x88A8][S-VID/PCP/DEI][0x8100][C-VID/PCP/DEI][EtherType/Length][Payload][FCS]
- Operational Workflow
- Ingress (Customer-to-SP Edge):
- A customer frame (single-tagged with C-VID) enters the SP network.
- The SP edge switch encapsulates the frame with an S-VID, creating a dual-tagged QinQ frame.
- Core Forwarding:
- SP core switches process only the outer S-VID for forwarding decisions, ignoring the inner C-VID.
- Enables efficient aggregation and scalability within the SP backbone.
- Egress (SP-to-Customer Edge):
- The egress SP switch strips the outer S-VID.
- The original customer frame (C-VID intact) is delivered to the destination.
- Use Cases
- Metro Ethernet Services: Delivers Layer 2 VPNs (E-LAN, E-LINE) to multiple tenants over shared infrastructure.
- Cloud Connectivity: Isolates tenant traffic in multi-tenant cloud environments.
- Cable Access Networks: Manages subscriber traffic in DSL/Cable broadband aggregation.
- Advantages Over 802.1Q
- Scalability: Eliminates VLAN ID exhaustion for SPs.
- Hierarchy: Separates customer and provider control planes.
- Compatibility: Backward-compatible with 802.1Q devices (outer tag processed as standard VLAN).
- Limitations and Considerations
- MTU Overhead: Adds 4 bytes per tag, requiring MTU adjustments to avoid fragmentation.
- Security: Relies on proper S-VID segregation; misconfiguration may cause cross-tenant leaks.
- Related Standards
- 802.1Q (Basic VLAN): Single tagging for local network segmentation.
- 802.1ah (MAC-in-MAC): Extends scalability further via customer MAC encapsulation (provider backbone bridging).
In summary, QinQ is a critical protocol for SPs to scale VLAN services while maintaining client isolation and operational simplicity. Its dual-tagging mechanism bridges enterprise and carrier networks, enabling flexible, large-scale Layer 2 service delivery.