What is MPLS (multiprotocol label switching)?

Multiprotocol label switching (MPLS) is a method for setting up dedicated paths across networks without relying on the typical routing process.

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  • Network Layer
  • How the Internet Works
  • Routing
  • What is a LAN?
  • Internet Protocol (IP)
  • Glossary

MPLS

Learning Objectives

After reading this article you will be able to:

  • Understand how multiprotocol label switching (MPLS) works
  • Contrast MPLS with the typical Internet routing process
  • Explore the uses for and drawbacks of MPLS

What is multiprotocol label switching (MPLS)?

Multiprotocol label switching (MPLS) is a technique for speeding up network connections that was first developed in the 1990s. The public Internet functions by forwarding packets from one router to the next until the packets reach their destination. MLPS, on the other hand, sends packets along predetermined network paths. Ideally, the result is that routers spend less time deciding where to forward each packet, and packets take the same path every time.

Consider the process of planning a long drive. Instead of identifying which towns and cities one must drive through in order to reach the destination, it is usually more efficient to identify the roads that go in the correct direction. Similarly, MPLS identifies paths — network "roads" — rather than a series of intermediary destinations.

MPLS is considered to operate at OSI layer "2.5", below the network layer (layer 3) and above the data link layer (layer 2).

How does routing normally work?

Anything sent from one computer to another over the Internet is divided up into smaller pieces called packets, instead of getting sent all at once. For example, this webpage was sent to your computer or device in a series of packets that your device reassembled and then displayed. Each packet has an attached header that contains information about where the packet is from and where it is going, including its destination IP address (like the address on a piece of mail).

For a packet to reach its intended destination, routers have to forward it from one network to the next until it finally arrives at the network that contains its destination IP address. That network will then forward the packet to that address and the associated device.

Before routers can forward a packet to its final IP address, they must first determine where the packet needs to go. Routers do this by referencing and maintaining a routing table, which tells them how to forward each packet. Each router examines the packet's headers, consults its internal routing table, and forwards the packet to the next network. A router in the next network goes through the same process, and the process repeats until the packet arrives at its destination.

This approach to routing works well for most purposes; most of the Internet runs using IP addresses and routing tables. However, some users or organizations want their data to travel faster over paths they can directly control.

How does routing work in MPLS?

In typical Internet routing, each individual router makes decisions independently based on its own internal routing table. Even if two packets come from the same place and are going to the same destination, they may take different network paths if a router updates its routing table after the first packet passes through. However, with MPLS, packets take the same path every time.

In a network that uses MPLS, each packet is assigned to a class called a forwarding equivalence class (FEC). The network paths that packets can take are called label-switched paths (LSP). A packet's class (FEC) determines which path (LSP) the packet will be assigned to. Packets with the same FEC follow the same LSP.

Each packet has one or more labels attached, and all labels are contained in an MPLS header, which is added on top of all the other headers attached to a packet. FECs are listed within each packet's labels. Routers do not examine the packet's other headers; they can essentially ignore the IP header. Instead, they examine the packet's label and direct the packet to the right LSP.

Because MPLS-supporting routers only need to see the MPLS labels attached to a given packet, MPLS can work with almost any protocol (hence the name "multiprotocol"). It does not matter how the rest of the packet is formatted, as long as the router can read the MPLS labels at the front of the packet.

Is an MPLS network a 'private' network?

MPLS can be "private" in the sense that only one organization uses certain MPLS paths. However, MPLS does not encrypt traffic. If packets are intercepted along the paths, they can be read. A virtual private network (VPN) does provide encryption and is one method for keeping network connections truly private.

What are the drawbacks of MPLS?

Cost: MPLS is more expensive than regular Internet service.

Long setup time: Setting up complicated dedicated paths across one or more large networks takes time. LSPs have to be manually configured by the MPLS vendor or by the organization using MPLS. This makes it difficult for organizations to scale up their networks quickly.

Lack of encryption: MPLS is not encrypted; any attacker that intercepts packets on MPLS paths can read them in plaintext. Encryption has to be set up separately.

Cloud challenges: Organizations that rely on cloud services may not be able to set up direct network connections to their cloud servers, as they do not have access to the specific servers where their data and applications live.

When is MPLS used?

MPLS can be used when speed and reliability are highly important. Applications that require near-immediate data delivery are known as real-time applications. Voice calls and video calls are two common examples of real-time applications.

MPLS can also be used to set up wide area networks (WANs). In recent years, software-defined WANs (SD-WANs) have emerged as a popular alternative to MPLS for WANs. To learn more about software-defined networking and SD-WANs, see What is software-defined networking (SDN)?