Virtual Switching Instance
Similar to VRF (virtual routing and forwarding instance) maintained in IP/MPLS routers to deliver layer 3 VPNs, IP VPNs, IP/MPLS switches maintain a Virtual Switching Instance (VSI) to deliver layer 2 VPNs, using VPLS.

As VRFs keep IP route entries for a particular L3 VPN, VSIs keep MAC address entries for a particular VPLS. MAC addresses are learned on pseudowires (an L2 switch learns MAC addresses on ports).