What is MPLS (Multiprotocol Label Switching)?

Multiprotocol Label Switching (MPLS) is a protocol-agnostic networking technology used to speed up and shape the flow of network traffic. It operates at the OSI model's Layer 2.5, sitting between the traditional Layer 2 (Data Link Layer) and Layer 3 (Network Layer). MPLS is designed to efficiently route data across networks, especially in large-scale, service-provider environments.

Here's a technical breakdown of MPLS:

  1. Label Switching:
    • Label: At the core of MPLS is the use of labels. A label is a short, fixed-length identifier (usually 20 bits) assigned to a packet by a router. This label is used to make forwarding decisions.
    • Label Switching Router (LSR): These are routers within the MPLS network capable of forwarding packets based on labels. LSRs are responsible for assigning labels to incoming packets and swapping or popping labels when forwarding packets.
  2. Label Distribution:
    • Label Distribution Protocol (LDP): MPLS uses a label distribution protocol to distribute labels between routers. LDP is the most common protocol for this purpose, but other protocols like RSVP (Resource Reservation Protocol) can also be used.
    • Label Forwarding Information Base (LFIB): Each LSR maintains a table called the LFIB, which contains mappings between incoming labels and the corresponding outgoing interfaces and labels.
  3. Packet Labeling Process:
    • Ingress Router: When a packet enters the MPLS network, the ingress router assigns a label to the packet based on its destination IP address. This label is then used to create a label stack.
    • Label Stack: MPLS allows for the stacking of multiple labels. The outermost label is used for routing within the MPLS network, while inner labels can represent various services or VPNs (Virtual Private Networks).
  4. Label Switching and Forwarding:
    • Label Switching: In the core of the MPLS network, LSRs make forwarding decisions based on the labels in the packet headers. This is much faster than traditional IP routing because it doesn't involve a detailed examination of the IP header.
    • Label Swapping and Popping: At each hop, the LSR examines the top label of the incoming packet, makes a forwarding decision based on its LFIB, and then either swaps the incoming label with a new one or pops the label off, revealing the next label in the stack or the original IP header.
  5. Benefits of MPLS:
    • Traffic Engineering: MPLS enables network operators to control and optimize the flow of traffic through the network.
    • Quality of Service (QoS): MPLS allows for the prioritization of traffic by assigning different labels to packets with varying QoS requirements.
    • Scalability: It simplifies routing and scales well for large networks, as it doesn't rely on complex IP routing tables.