MPLS VPN

MPLS VPN is a solution to solve duplicate customer address range in the provider routing table. By having this solution, a service provider will be able to provide a connectivity for several customers which are connected each other with privete IP(s).

Data plane on this solution is a bit different with MPLS unicast. The PE (provider edge) has another table called VRF (Virtual Routing and Forwarding). The PE also add two labels (instead of one label) into the header. The outer label is being used to deliver the packet to the next router, while the inner label is being used to identify which VRF is related to the packet.

One of the challange to implement VPN is overlapping IP address. This problem is solved by using RD (route distinguisher) by taking advantage of MP-BGP. The RD will add 64-bit RD to the 32-bit IP-v4  prefix. Then MPLS will your RT (route target) to decide which packet is correlated to which VRF.

MPLS Unicast IP

MPLS is a protocol that forward packet based on Label instead of IP address. There are several MPLS application, and one of them is MPLS Unicast IP. A router with MPLS Unicast IP is actually forwarding a packet to the same path as a router with IP routing. The difference is only that the MPLS router forwards the packet based on Label. Then why it is important to know what MPLS Unicast IP is? It is important since many of the application is using MPLS unicast IP forwarding as part of the MPLS network.

MPLS has something called Control Plane and Data Plane. The Data Plane are the data structured used by the MPLS in operation.  FIB, LFIB, LIB are the tables that we are talking about. FIB contains information to map IP address and network interface of where a packet destined to an IP address will be forwarded. LFIB contains information to map label and network interface of where a packet with a label will be forwarded. And the LIB is actualy a library of LFIB, which map all labels and ports including the ones which are not currently being used to forward packet. LIB contains only the best map, so that there is only network interface mapped to a label on it. 

Now, how the LIB feeds the LFIB and FIB? 

In simple words, the MPLS routers distribute routing information using EIGRP, OSPF or other routing procotol. From this protocol, the MPLS router will know where is the best route to delivere a packet. The router will pick up the best route, find the related label on the LIB and then update the LFIB.