Virtual Private Network (VPN) is exactly that - a virtual connection over a shared medium that allows customer traffic to be separate from traffic of other customers. It's not a new concept - ATMs, Frame Relay and leased line work on the same basic principle. Nowadays however, VPNs refer to the general idea, not a specific technology of transferring packets. Most common use today is interconnecting multiple locations over an IP based packet switching/routing technology. There are few ways of going about it - ones that do not interact with customer's routing domain and ones that do:
- L3VPN - MPLS based method that interacts with customer over layer 3 (shared medium becomes a logical router that interconnects multiple locations).
- L2VPN - MPLS based method that interacts with customer over layer 2 (shared medium becomes a logical switch - customer takes care of routing on their own).
Basic definitions:
- Customer Edge (CE) - router physically located in customer's location. Forms routing protocol adjacency with provider's side for exchange of customer's routes between interconnected locations or uses static routes.
- Provider Edge (PE) - router belonging to a provider which encapsulates customer's data and sends it over the provider's network along with redistributing customer's routes to other PEs connected to customer's locations.
- Provider Router (P) - routers in provider's core that don't peer with customers and just forward MPLS labeled customer data.
- VPN Routing and Forwarding table (VRF) - every customer gets its own routing table, that stores received routes. Contents of VRF table are then advertised to other PE routers that are connected to locations belonging to the same customer.
