MPLS

networkingISP & telecom

Multiprotocol Label Switching - forwarding by short labels instead of IP lookups: push a label at the edge, swap it hop by hop, pop it before the exit. The machinery under most carrier services.

An edge router classifies a packet once, pushes a label, and the core simply swaps labels down a pre-built label-switched path - no core router needs the internet's routing table (the BGP-free core), and labels can stack, which is the whole trick: an inner label meaning 'this customer's VPN' rides an outer label meaning 'this path across the backbone'. That stack is how L3VPNs (RFC 4364) keep thousands of customers' overlapping address spaces separate on one network, and how traffic engineering pins flows to explicit paths. Labels historically came from LDP or RSVP-TE; Segment Routing now encodes the path in the packet and retires the signaling. MPLS took ATM's virtual-circuit ideas and left the cells behind - and while SD-WAN reshaped the enterprise edge, the carrier core underneath still speaks labels.

MPLS forwards packets on a short label rather than by looking up a destination address, and the original justification was speed on hardware that found longest-prefix matching expensive. That justification evaporated as silicon improved, and MPLS thrived anyway, because the labels turned out to be useful for something else entirely.

What it actually delivers is traffic engineering and separation. Because a label defines a path rather than a destination, an operator can route traffic along a chosen route instead of the shortest one, reserving capacity and steering around congestion. Stacking labels allows a carrier to run many customers' private networks across one shared backbone without any of them seeing each other, which is what an MPLS VPN service really is.

Its position today is genuinely contested. SD-WAN made it possible to build resilient connectivity over ordinary internet links at a fraction of the price, and plenty of enterprises have migrated away. What MPLS still sells is a contractual performance guarantee rather than best-effort delivery, which matters for latency-sensitive traffic and matters much less for a workload already sitting in a public cloud.

Also known as: label switching, lsp, ldp, l3vpn, segment routing

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