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Unpacking MPLS: A Technical Overview of Multi-Protocol Label Switching

21 September 2023
Melanie Gomersall

Trusted by:

Telecom Egypt
BC Hydro


National Grid
Open Fiber
TPX Communications
Ella Link
Red Iris
Surf Net

In the realm of complex network architectures, Multi-Protocol Label Switching (MPLS) occupies a significant position as a robust routing mechanism. Designed to expedite and optimize the flow of network traffic, MPLS is often favoured for its efficiency over traditional IP routing. This article provides an in-depth analysis of MPLS—its architecture, advantages, connection types, the relationship with WDM, vendors and how it distinguishes itself from traditional IP-based routing methodologies.

What is MPLS?

MPLS, or Multi-Protocol Label Switching, is a scalable, protocol-independent data-carrying service. Unlike traditional IP routing that relies heavily on examining the packet header to make forwarding decisions, MPLS uses labels to determine packet forwarding. These labels, simple numeric identifiers, are added to data packets to instruct Label Switch Routers (LSRs) in the network on how to forward these packets.

MPLS, or Multi-Protocol Label Switching, is a scalable, protocol-independent data-carrying service. Unlike traditional IP routing that relies heavily on examining the packet header to make forwarding decisions, MPLS uses labels to determine packet forwarding. These labels, simple numeric identifiers, are added to data packets to instruct Label Switch Routers (LSRs) in the network on how to forward these packets.

MPLS Architecture

The core components of an MPLS network include Label Edge Routers (LERs) and Label Switch Routers (LSRs). LERs sit at the boundary of the MPLS network and are responsible for adding and removing MPLS labels. LSRs, on the other hand, are internal routers that switch labels to forward packets through the network. They use a Forwarding Equivalence Class (FEC) to decide how to forward packets.

MPLS vs Traditional Routing

Computational Efficiency
In traditional IP routing, routers parse through the Network Layer header to decipher the destination IP address, which is then matched against complex routing tables to forward the packet. This process is computationally intensive and slows down data packet transmission. MPLS circumvents this by employing labels, which are pre-assigned, making the look-up process simpler and more efficient.

Deterministic Routing
Traditional IP routing uses dynamic routing algorithms that can change the path of a packet depending on network conditions. While this offers flexibility, it can lead to jitter and latency in real-time applications. MPLS networks use Label Switched Paths (LSPs), which are pre-established paths that offer deterministic routing, reducing latency and jitter.

QoS and Traffic Engineering
Traditional IP networks may offer Quality of Service (QoS), but it’s often more complicated to implement across diverse infrastructures. MPLS inherently supports QoS and allows for more granular levels of traffic engineering, making it easier to prioritize specific types of traffic.

Label Switching in Action

Label Assignment
When a data packet enters the MPLS network, the ingress LER assigns an MPLS label based on the FEC. The FEC is determined through various attributes like the source and destination IP address, IP protocol ID, and even port numbers.

Label Swapping
As the labeled packet traverses through the MPLS network, each LSR examines the label, swaps it for a new label based on its label forwarding table, and forwards the packet to the next hop. This action is repeated until the packet reaches the egress LER.

Label Removal
The egress LER removes the MPLS label before the packet exits the MPLS network, converting it back to a standard IP packet, which is then routed to its final destination.

What is MPLS

MPLS Connection Types

MPLS offers multiple connection types to serve different networking needs, primarily falling into two categories: Point-to-Point and Point-to-Multipoint.

Point-to-Point (Pseudowire)
In a Point-to-Point connection, also known as a Pseudowire, data packets are transported between two endpoints. This is often used for connecting two remote LAN segments over an MPLS backbone.

Point-to-Multipoint (MP2MP)
Point-to-Multipoint (MP2MP) connections involve one source sending data to multiple endpoints. This is useful in applications like IPTV broadcasting where a single source needs to distribute data to multiple receivers.

Whilst MPLS primarily uses these two fundamental connection types for transporting data: Point-to-Point (Pseudowire) and Point-to-Multipoint (MP2MP), there are however variations and subtypes within these categories.

Here are some additional types:

Layer 2 MPLS VPNs

  1. VPLS (Virtual Private LAN Service): VPLS allows multiple sites to be connected over a single bridged domain, essentially acting like a single Ethernet switch.
  2. MPLS-TP (MPLS-Transport Profile): MPLS-TP is a simplified version of MPLS designed for transport networks. It offers LSPs (Label Switch Paths) but lacks some of the more advanced features of standard MPLS like traffic engineering.

Layer 3 MPLS VPNs

  1. L3VPN: In Layer 3 VPNs, the MPLS label is added to packets along with the routing information. This allows the service provider to route packets through their network without knowing the details of the customers’ internal networks.
  2. MPLS Multicast VPNs: In a multicast VPN, MPLS labels are used to distribute multicast traffic to multiple points across the MPLS network.

Traffic Engineering Tunnels

  1. RSVP-TE (Resource Reservation Protocol with Traffic Engineering): This allows the creation of reserved paths for particular data flows, enabling better bandwidth utilization and handling of Quality of Service (QoS) parameters.
  2. LD-TE (Constraint-Based Routing Label Distribution Protocol): This is an alternative to RSVP-TE, used for setting up pre-determined paths within an MPLS network based on constraints like bandwidth and hop-counts.

Each of these types has its own unique applications, advantages, and limitations. Some are better suited for small, simple networks, while others are designed for large, complex, multi-site implementations.

Relationship between MPLS and WDM

Wavelength Division Multiplexing (WDM) is another technology that significantly influences modern networking. WDM allows multiple optical signals to be sent over a single fiber strand. MPLS and WDM often work in concert, providing both the transport layer (WDM) and the service layer (MPLS) in a network.

Multi-Layer Networking
MPLS can sit on top of a WDM network to provide a multi-layer networking solution. In this setup, WDM takes care of the optical layer while MPLS handles the data layer. This results in optimized bandwidth utilization, increased network efficiency, and lowered operational costs.

Traffic Engineering
Both MPLS and WDM offer traffic engineering features, but they operate at different layers. MPLS can handle the prioritization of data packets, while WDM can allocate specific wavelengths for different data types or customers.

MPLS and Vendors

MPLS solutions are provided by various networking vendors, including VC4, each with its own set of features, advantages, and limitations. Some providers offer robust and scalable solutions, which integrate seamlessly with a wide array of networking hardware. Others, offer user-friendly configuration options, whilst other still, are long standing players in the MPLS domain, and offer reliability.

Implications for the Future

With current technologies like 5G, IoT, and SD-WAN becoming more prevalent, MPLS’s role as an efficient, reliable routing mechanism is likely to grow. Its ability to support deterministic routing and advanced QoS features will be critical in managing increasingly complex network demands.
MPLS offers a plethora of benefits over traditional IP routing methods. By replacing cumbersome address-based routing with more efficient label-based switching, MPLS provides a streamlined, effective solution for complex network topologies. Its role in future network architectures, particularly with the advent of technologies that require rapid, reliable data transmission, is undeniably important. Therefore, understanding the nuances of MPLS is crucial for network engineers and architects involved in designing next-generation networks.

VC4 and MPLS

The integration of VC4-IMS with MPLS networks brings about a synergy that combines the robustness of MPLS routing with the intelligence of a sophisticated network inventory management solution. This results in a highly efficient, manageable, and scalable network infrastructure. Given the rapidly evolving networking landscape, leveraging technologies like MPLS and VC4-IMS software is increasingly becoming a necessity for organizations aiming for operational excellence in telecom network management.