Layer 3 VPN Service is a service that connects multiple branches in a single logical routed architecture over IP/MPLS network of a Service Provider. .
Different brances of a customer, pass through the IP/MPLS network of Service Provider and this Layer 3 VPN infrastructure seems to the customer as their branches are connected to a single company Router.
Many different customer networks exist in this IP/MPLS network. And these are managed by Service Provider. Each of the customer in IP/MPLS network does not know the other network and use the Service Provider managed IP/MPLS network as their own. This provides indipendence to the customer and efficient usage of network to the Service Provider.
With L3 VPN, multiple sites of the customer can connect over the IP/MPLS Network of Service Provider. So, from the customer perspective, the sites are connected to a Router.
You can see the logical view of L3 VPN below:
In L3 VPN, IP addresses are important, because it works on Layer 3.The traffic is routed according to IP addresses. This is the same logic used by routers. Think about this, a mini router in the main Servis Router.
From the customer perspective, L3 VPNservice seem like below:
There are many benefits of L3 VPN Service. These benefits are for both customer and Service Provider. These benefits are mentions one by one below:
• Service Provider can serve multiple customers over a single IP/MPLS network
• Customers can use any private IP subnets
• By using Service Provider IP/MPLS network, customers provide Redundancy easily
• Customers benefit using a Service Privder managed IP/MPLS network without extra effors
The routers used in L3 VPN Service is mainly the same routers used in IP/MPLS networks. We have also talked about them before. Here, as a summary, let’s remember these devices.
• Provider Router (P) : Service Provider Core Routers. Unaware of service. Only provides transport with MPLS Label Swapping.
• Provider Edge Router (PE) : Service Provider Core Routers that are towards Customer. Services are defined on these routers. PE Routers exchange L3 VPN routes with other PE Routers.
• Cusomer Edge Router (CE) : Customer Routers that towards to Service Provider Edge Routers.
As an example, you can check the below picture. In this picture, there are two customers each has two sites. L3 VPN 1 service is for Customer 1 and L3 VPN 2 service is for Customer 2.Each site has their own ip block and one block is overlapped as you see. Overlapped blocks can be used by different Customers in different L3 VPN service i as we said before. And below, you can also see the VRF Tables of PE routers.