HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 2 - LAN Switching Co - Page 163

Configuring QinQ Throughout this document, customer network VLANs (CVLANs), also called inner VLANs, refer to the VLANs that a customer uses on the private network; and service provider network VLANs (SVLANs), also called outer VLANs, refer to the VLANs that a service provider uses to carry VLAN tagged traffic for customers. Overview 802.1Q-in-802.1Q (QinQ) is a flexible, easy-to-implement Layer 2 VPN technology based on IEEE 802.1Q. QinQ enables the edge device on a service provider network to insert an outer VLAN tag in the Ethernet frames from customer networks, so that the Ethernet frames travel across the service provider network (public network) with double VLAN tags. QinQ enables a service provider to use a single SVLAN to serve customers who have multiple CVLANs. Background and benefits The IEEE 802.1Q VLAN tag uses 12 bits for VLAN IDs. A device supports a maximum of 4094 VLANs. This is far from enough for isolating users in actual networks, especially in metropolitan area networks (MANs). By tagging tagged frames, QinQ expands the available VLAN space from 4094 to 4094 × 4094. QinQ delivers the following benefits: • Releases the stress on the SVLAN resource. • Enables customers to plan their CVLANs without conflicting with SVLANs. • Provides an easy-to-implement Layer 2 VPN solution for small-sized MANs or intranets. • Enables the customers to keep their VLAN assignment schemes unchanged when the service provider upgrades the service provider network. How QinQ works The devices in the public network forward a frame only according to its outer VLAN tag and obtain its source MAC address into the MAC address table of the outer VLAN. The inner VLAN tag of the frame is transmitted as the payload. 154