• Multiprotocol Label Switching (MPLS) is a packet-forwarding method defined in RFC 3031.
    Instead of making forwarding decisions based on the destination IP address at every single hop, routers forward packets using a short numerical identifier called a label.

    Virtually every Service Provider and large enterprise core network runs MPLS. Before looking at the label structure itself, let's examine the problem it originally solved and why it remains vital today.

    Traditional IP Routing

    Look at the topology below: a packet leaves 192.168.1.0/24, crosses five routers, and reaches a destination host in 192.168.2.0/24.

    Diagram showing traditional IP routing with Layer 3 lookup on every router hop

    Figure 1 – Traditional IP routing (hop-by-hop)

    In a pure IP network, every router along the path performs a longest-prefix match lookup in its routing table (FIB) for every single packet.
    In large core networks carrying full Internet routing tables (over 900,000 routes), this creates massive memory and lookup overhead on every router.

    Answer the question below

    Which operation does every router repeat on the packet?

    MPLS Label Switching

    MPLS keeps the initial lookup at the edge and eliminates IP routing lookups in the core.

    Diagram illustrating MPLS label lookup mechanism reducing router CPU processing

    Figure 2 – MPLS label switching concept

    When a packet enters the network, R1 performs one initial IP lookup. Instead of simply forwarding the plain IP packet, it attaches (pushes) a short numerical label to the packet header.

    From that point forward, intermediate routers (R2, R3, R4, and R5) forward the packet based strictly on the label:

    • A label is a simple, fixed-length number.

    • Instead of performing a complex longest-prefix match against an IP routing table, each core router performs a single, fast exact match lookup on that label value.

    Answer the question below

    Which router performs the IP lookup when the packet enters the network?

    The Benefits of MPLS

    MPLS brings three practical benefits over plain IP routing:

    Benefit

    What it gives you

    Lightweight core

    The core forwards on labels alone, without inspecting the IP header.

    Multiprotocol

    The label can carry almost anything: IPv4, IPv6, or Layer 2 frames like Ethernet.

    Services

    VPNs, traffic engineering, and QoS built directly on the label.

    Table 1 – The three benefits of MPLS

    That last row is the focus of this module: the MPLS Layer 3 VPN you will build at the end relies entirely on this label mechanism.

    Answer the question below

    A provider wants to sell VPNs on top of its backbone. Which of the three benefits makes that possible?

    Answer the question below

    In the MPLS approach, what does R3 read to forward the packet?