A key to the success of MPLS and the L3 MPLS VPN is the use of "tunnels" created by the MPLS labeling. Tunnelling in the service provider cloud has many benefits:
- Only edge points (ingress and egress) need to understand the meaning of the inner network information (prefixes); core routers simply switch traffic based on labels
- You can easily re-direct tunnel traffic explicitely
- Tunnels can be created within tunnels
- The tunnel is less prone to data spoofing
- The overhead with MPLS is relatively low (4 bytes per MPLS header)
Notice how edge routers are known as Label Edge Routers (LERs) or Provider Edge Routers (PEs). Routers in the core of the provider network are called Label Switching Routers (LSRs) or Provider (P) routers. Label Switched Paths (LSPs) represent the path traffic takes through the provider MPLS network. All of the traffic that is to be forwarded using the same path is known as the Forwarding Equivalence Class (FEC). All of this traffic is forwarded with the same MPLS label. In the simple case of the Routing and Switching CCIE Exam, the FEC typically consists of all packets with a destination address of the BGP next-hop.
Notice how the network is a thing of beauty for the Label Switching Routers (LSRs)/Provider (P) routers, especially when you consider scalability. The service provider can add more customers and introduce many more network prefixes into its infrastructure, but these prefixes only need to exist on Label (Provider) Edge Routers (LERs/PERs). These edge devices are the "workhorses" of the provider network. Notice among many jobs, it is their responsibility to identify the Label Switched Path (LSP) that the packet is destined for.
Customer networks consist of Customer Edge routers (CEs) and Customer (C) routers. These devices need no knowledge whatsoever about MPLS. They can be completely oblivious to the fact they are interacting with a structure that uses MPLS as its basis for forwarding traffic.
In order to master MPLS for the CCIE R&S written and lab exam, this is the first of the terminology we must commit to memory regarding MPLS. Please join me for future posts in this series.