Multi Protocol Label Switching (MPLS)

Multi-Protocol Label Switching (MPLS) is an advanced packet-forwarding technique used in modern networks. Instead of making routers look into complex Layer 3 routing tables for every IP packet, MPLS uses labels for forwarding decisions. These labels create pre-defined, efficient paths across the network, which enhances speed, scalability and traffic management.

Note: MPLS lies between Layer 2 (Data Link) and Layer 3 (Network Layer) of the OSI model, which is why it is often referred to as a Layer 2.5 protocol.

Why MPLS

• Traditional IP routing involves a long lookup process in routing tables.
• MPLS simplifies forwarding by assigning a short fixed-length label to each packet.
• This ensures: Faster packet delivery, better support for Quality of Service (QoS) & easier traffic engineering and Virtual Private Networks (VPNs).

MPLS Header

The MPLS header is 32 bits long and is inserted between the Layer 2 and Layer 3 headers. It contains the following fields:

Note: Multiple labels can be stacked to form a label stack, enabling hierarchical routing.

Key Terminologies in MPLS

• Provider Edge (PE) Router: Router at the edge of the MPLS provider’s network; adds/removes labels from packets.
• Customer Edge (CE) Router: Router at the customer’s network edge that communicates with PE routers.
• Label Switch Router (LSR): Router inside the MPLS core that understands and processes labels.
• Ingress LSR: First router that receives the packet from CE and pushes (adds) the MPLS header.
• Intermediate LSR: Routers that swap labels as packets move across the MPLS path.
• Egress LSR: The last router in the MPLS domain that pops (removes) the MPLS header before sending the packet to the CE.
• Push, Pop, Swap: Push -> Add a label, Pop -> Remove a label. & Swap -> Replace an existing label with a new one.

Working of MPLS (Forwarding Process)

• Ingress Stage (Push): CE sends an IP packet to the PE (Ingress LSR). Ingress PE assigns a label based on the destination and attaches an MPLS header.
• Core Stage (Swap): Intermediate LSRs forward the packet based only on the label. They swap the label with a new one as defined in their Label Forwarding Information Base (LFIB).
• Egress Stage (Pop): The Egress PE removes the MPLS header and forwards the original IP packet to the CE router.

Note: Labels are distributed using the Label Distribution Protocol (LDP).

MPLS and VPN

• MPLS can separate traffic of different customers similar to VPNs.
• Unlike regular VPNs, MPLS does not encrypt data.
• Instead, it ensures traffic isolation by keeping each customer’s traffic within its label-switched path (LSP).

Read more about Difference between VPN and MPLS

Advantages of MPLS

• Faster packet forwarding (label-based).
• Supports multiple protocols (hence “multi-protocol”).
• Enables Traffic Engineering (efficient use of resources).
• Facilitates Quality of Service (QoS).
• Provides scalable support for Layer 3 VPNs.
• Reliable with loop prevention (via TTL).

Disadvantages of MPLS

• Expensive to implement compared to IP routing.
• Complexity in configuration and management.
• Security is weaker than encrypted VPN solutions.
• Less suitable for small-scale networks.
• Multiprotocol Label Switching (MPLS) Routing.