208Application scenario of one-to-two and two-to-two VLANmappingFigure 56 shows a typical application scenario of one-to-two and two-to-two VLAN mapping. In thisscenario, the two remote sites of the same VPN must communicate across two SP networks.Figure 56 Application scenario of one-to-two and two-to-two VLAN mappingSite 1 and Site 2 are in VLAN 2 and VLAN 3, respectively. The SP 1 network assigns SVLAN 10 to Site1. The SP 2 network assigns SVLAN 20 to Site 2. When the packet from Site 1 arrives at the edge ofnetwork SP 1, PE 1 tags the packet with SVLAN 10 by using one-to-two VLAN mapping.When the double-tagged packet from the SP1 network arrives at the SP 2 network interface, PE 3processes the packets as follows:• Replaces SVLAN tag 10 with SVLAN tag 20.• Replaces CVLAN tag 2 with CVLAN tag 3.One-to-two VLAN mapping provides the following benefits:• Enables a VPN user to plan the VLAN IDs in the network without conflicting with SVLANs.• Adds a VLAN tag to a tagged packet and expands the number of available VLANs to 4094 ×4094.• Reduces the stress on the SVLAN resources, which were 4094 VLANs in the SP network before themapping process was initiated.VLAN mapping implementationsFigure 57 shows a simplified network to help explain the concepts and terms in VLAN mapping.These basic concepts include the following:• Uplink traffic—Traffic transmitted from the customer network to the service provider network.• Downlink traffic—Traffic transmitted from the service provider network to the customer network.• Network-side port—A port connected to or closer to the service provider network.• Customer-side port—A port connected to or closer to the customer network.
