SDN Blog

MPLS: Layer 2.5?

Posted on Tuesday, November 10, 2009

Blog written by SDN Communications

MPLS – or Multi-Protocol Label Switching—definitely falls within the seven layers of the OSI (Open Systems Interconnection Reference) Model. But when you consider its function, does it perform as Layer 2 or Layer 3? It’s probably closer to Layer 2.5.

MPLS is a method used to increase the speed of network traffic flow by inserting information about a specific path the packet is taking en route to its destination. This saves the time needed for a router to look up the address for the next node that the packet is supposed to be sent to.

How else is MPLS more efficient?

Penultimate Hop Popping. It’s the process that allows the outermost label of an MPLS tagged packet to be removed—saving time and preventing the need to perform at least two additional label lookups before the packet is passed on to a Label Edge Router (LER) and delivered. MPLS works with ATM, Ethernet, Frame Relay, HDLC and PPP communication methods. It’s an inter-office transport method that allows, unlike connectivity methods to be able to talk to each other through one fully-meshed cloud.

So, where does it fit into the OSI Model?

Layer 2 of the OSI Model, the Data Link Layer, provides the means to transfer data between network entities. Generally, only error detection, not flow control, is present in protocols like PPP.

Layer 3, the Network Layer, provides the network routing functions that transfer variable length data sequences from a course to a destination via one or more networks while maintaining QoS, or Quality of Service.

The best example of Layer 3 is Internet Protocol or IP. A single IP/MPLS network infrastructure can carry IP traffic in Layer 3 VPNs. While point-to-point virtual circuits are built with MPLS features, the Layer 2 connections remain characteristic of VPNs. MPLS is independent of Layer 2 and Layer 3 protocols such as ATM and IP.

Since MPLS technology uses Ethernet, PPP and IP protocols to provide a unified data-carrying service for both circuit-based users and packet-switching users, can it be considered Layer 2.5?