What is MPLS?

Multi-Protocol Label Switching (MPLS) defines a mechanism for packet forwarding in network routers. It was originally developed to provide faster packet forwarding than traditional IP routing, although improvements in router hardware have reduced the importance of speed in packet forwarding. However, the flexibility of MPLS has led to it becoming the default way for modern networks to achieve Quality of Service (QoS), next generation VPN services, and optical signaling.

Traditional IP networks are connectionless: when a packet is received, the router determines the next hop using the destination IP address on the packet alongside information from its own forwarding table. The router's forwarding tables contain information on the network topology. They use an IP routing protocol, such as OSPF, IS-IS, BGP, RIP or static configuration, to keep their information synchronized with changes in the network.

MPLS also uses IP addresses, either v4 or v6, to identify end points and intermediate switches and routers. This makes MPLS networks IP-compatible and easily integrated with traditional IP networks. However, unlike traditional IP, MPLS flows are connection-oriented and packets are routed along pre-configured Label Switched Paths (LSPs).

How does MPLS work?

MPLS works by tagging packets with an identifier (a label) to distinguish the LSPs. When a packet is received, the router uses this label (and sometimes also the link over which it was received) to identify the LSP. It then looks up the LSP in its own forwarding table to determine the best link over which to forward the packet, and the label to use on this next hop.

A different label is used for each hop, and it is chosen by the router or switch performing the forwarding operation. This allows the use of very fast and simple forwarding engines, as the router can select the label to minimize processing.

Ingress routers at the edge of the MPLS network use the packet's destination address to determine which LSP to use. Inside the network, the MPLS routers use only the LSP labels to forward the packet to the egress router.

In the diagram above, LSR (Label Switched Router) A uses the destination IP address on each packet to select the LSP, which determines the next hop and initial label for each packet (21 and 17). When LSR B receives the packets, it uses these labels to identify the LSPs, from which it determines the next hops (LSRs D and C) and labels (47 and 11). The egress routers (LSRs D and C) strip off the final label and route the packet out of the network.

As MPLS uses only the label to forward packets, it is protocol-independent, hence the term "Multi-Protocol" in MPLS. Packet forwarding has been defined for all types of layer-2 link technologies, with a different label encoding used in each case.

MPLS-enabled VPN Services

VPN and VLAN technology is becoming increasingly important as more workers require remote network access and as companies outsource their internal network infrastructure to service providers. To satisfy this market, service providers need a scalable VPN solution, and MPLS, with its ability to separate flows into logical streams, makes it an ideal technology to deploy. With an MPLS backbone network, a provider has the flexibility to offer a wide range of high-value VPN services.

MPLS-enabled VPN solutions can be categorized by visualizing a network that includes Customer Edge (CE) and Provider Edge (PE) devices, as below.
The different VPN solutions provide the CE with a different perception of its connection to the PE.