8021ad

IEEE 802.1ad, commonly known as Q-in-Q or Provider Bridges, is an IEEE standard that extends the VLAN tagging capabilities of 802.1Q. It enables an Ethernet frame to carry an additional VLAN tag, effectively layering customer VLANs inside a provider VLAN. This allows service providers to transport numerous customer VLANs across their networks while presenting a single outer VLAN to the rest of their infrastructure.

In practice, 802.1ad introduces two stacked VLAN tags. The inner tag, or C-tag, identifies the customer VLAN, while the outer tag, or S-tag, identifies the provider’s VLAN. The outer tag uses a different tag protocol identifier (TPID) from the inner tag, typically 0x88A8 for the outer tag and 0x8100 for the inner tag, creating a double-tagged frame. This outer tagging enables service providers to segregate and route customer traffic without altering the customer’s VLAN identifiers.

Applications and deployment of 802.1ad are common in service-provider networks, especially metro Ethernet and other environments requiring scalable VLAN separation. It also helps data centers and VPN deployments that need to extend customer VLANs across larger, multi-tenant networks.

Relation to other standards: 802.1ad builds on IEEE 802.1Q tagging and is often discussed alongside 802.1ah (Provider Backbone Bridges), which further scales networks through MAC-in-MAC encapsulation to reduce MAC learning loads in large backbones.

Limitations and considerations include that 802.1ad is a tagging mechanism, not a security feature, and correct configuration is essential to avoid mis-tagging or interoperability issues.