144QinQ configurationThis chapter includes these sections:• Introduction to QinQ• QinQ configuration task list• QinQ configuration examplesNOTE:Throughout this document, customer network VLANs (CVLANs), also called "inner VLANs", refer to theVLANs that a customer uses on the private network; service provider network VLANs (SVLANs), alsocalled "outer VLANs", refer to the VLANs that a service provider uses to carry VLAN tagged traffic forcustomers.Introduction to QinQQinQ stands for 802.1Q in 802.1Q. QinQ is a flexible, easy-to-implement Layer 2 VPN technologybased on IEEE 802.1Q. QinQ enables the edge switch on a service provider network to insert an outerVLAN tag in the Ethernet frames from customer networks, so that the Ethernet frames travel across theservice provider network (public network) with double VLAN tags. QinQ enables a service provider touse a single SVLAN to serve customers who have multiple CVLANs.Background and benefitsThe IEEE 802.1Q VLAN tag uses 12 bits for VLAN IDs. A switch 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. QinQdelivers 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.• Allows the customers to keep their VLAN assignment schemes unchanged when the service providerupgrades the service provider network.How QinQ worksThe switches in the public network forward a frame only according to its outer VLAN tag and learn itssource MAC address into the MAC address table of the outer VLAN. The inner VLAN tag of the frame istransmitted as the payload.
