Introduction to Multiprotocol Label Switching

Multiprotocol Label Switching (MPLS) is a relatively new technology as far as WAN technologies are concerned. MPLS offers a paradigm shift in the way we think about forwarding packets across the WAN. Where traditional WAN technologies such as Frame-Relay are predominantly Layer 2 architectures, MPLS can extend Layer 3 functionality across the WAN, effectively extending the enterprise network across the WAN.

There are two main components to the Multiprotocol Label Switching architecture. The forwarding component uses a label-switching database to forward packets based on labels carried by packets. The control component is responsible for creating and maintaining label-forwarding information (bindings) among a group of connected label switches.

Multiprotocol Label Switching prepends labels to packets as they enter the MPLS WAN. The labels applied to the packets are determined by classifying the packets into a Forwarding Equivalence Class (FEC). Each Forwarding Equivalence Class is mapped to a next hop. Once a FEC is assigned to a packet, no further layer 3 analysis needs to be performed while in the MPLS domain. All packets belonging to a FEC will follow the same path (or in some cases the same set of paths) through the MPLS network.

When these packets are forwarded through the Multiprotocol Label Switch network, forwarding decisions can be made upon the encoded label rather than the network layer headers. It is the use of these labels that allow Multiprotocol Label Switching to maintain the strong and fast characteristics of traditional WAN technologies, while providing the robust traffic engineering policies afforded layer 3 routing protocols.

Conventional packet forwarding has to rely on network layer information to make forwarding decisions. MPLS can classify packets into FECs using ANY information available about the packet, including the interface in which the packet entered the network. This can be done even if no network information can be retrieved from existing layer 3 headers. This allows packets destined for the same network to be assigned different labels which can then be used for complex traffic engineering and routing policies. That being said, it goes without saying that these strong FEC classification abilities allow MPLS to classify packets based upon the ingress router, supporting routing policies that depend on the ingress router.

That is the very brief overview of Multiprotocol Label Switching. In MPLS Part II we'll get some important terminology out of the way, before we dive into Label Switch Routers (LSR) and the types of LSRs you're likely to encounter in the realm of MPLS, followed by an exciting and informative lab detailing the configuration of a Frame Mode MPLS network.