Understanding Multiprotocol Label Switching
Multiprotocol Label Switching (MPLS) is a data-carrying technique that directs traffic based on short labels rather than long network addresses, avoiding complex lookups in routing tables. MPLS inserts a 32-bit label between the Layer 2 (Ethernet) and Layer 3 (IP) headers. Routers forward packets based on this label, which identifies a pre-determined path (Label Switched Path or LSP) through the network.
How MPLS Works
In traditional IP routing, each router independently looks up the destination IP address in its routing table and forwards the packet hop-by-hop. With MPLS, the first router (Label Edge Router or LER) classifies the packet, assigns it to a Forwarding Equivalence Class (FEC), and pushes a label onto it. Subsequent routers (Label Switch Routers or LSRs) simply swap the label and forward—a much faster operation that avoids full IP lookups at every hop.
MPLS Services
- Layer 3 VPN (L3VPN): Defined in RFC 4364, L3VPN enables service providers to offer fully meshed, any-to-any connectivity between customer sites using BGP/MPLS. The provider edge (PE) router maintains separate Virtual Routing and Forwarding (VRF) tables for each customer, ensuring complete traffic isolation. This is the most common enterprise MPLS service.
- Virtual Private LAN Service (VPLS): Extends a Layer 2 broadcast domain across geographically distributed sites, making them appear to be on the same LAN. VPLS uses MPLS pseudowires to tunnel Ethernet frames between sites.
- Pseudowires: Point-to-point Layer 2 connections that emulate T1/E1, ATM, Frame Relay, or Ethernet circuits across an MPLS network. Useful for migrating legacy services onto a unified MPLS backbone.
MPLS vs SD-WAN: The Great Debate
MPLS offers guaranteed Quality of Service with carrier-backed SLAs—99.999% availability, sub-50ms failover, end-to-end jitter guarantees, and predictable latency. These characteristics make MPLS the gold standard for real-time applications: voice over IP, video conferencing, financial trading platforms, and telemedicine.
SD-WAN, however, changes the value proposition. By bonding multiple low-cost broadband circuits with intelligent path selection, SD-WAN can match MPLS reliability at 50-70% lower cost. Organizations are increasingly adopting a hybrid model: MPLS for mission-critical, latency-sensitive applications and broadband (managed by SD-WAN) for everything else. For most enterprises, MPLS remains relevant but is no longer the default choice for every connection.
