1 · MPLS Architecture & Header Mechanics¶

Multiprotocol Label Switching (MPLS) replaces slow, computationally expensive 32-bit IPv4 / 128-bit IPv6 longest-prefix match (LPM) routing lookups at every intermediate router with fast $O(1)$ hardware label swapping.

The 32-Bit MPLS Shim Header Format¶

An MPLS label header (often called a shim header) is inserted between the Layer 2 Data-Link header (e.g. Ethernet EtherType 0x8847 for unicast, 0x8848 for multicast) and the Layer 3 IP payload.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                Label Value (20 bits)          | TC  |S|  TTL  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                (3b) (1b) (8b)

Field Breakdown¶

Field	Bit Length	Description
Label Value	20 bits	Range: `0 – 1,048,575`. Identifies the Forwarding Equivalence Class (FEC).
Traffic Class (TC / Exp)	3 bits	QoS / CoS priority markings (formerly Experimental bits). Maps 1:1 to IP DSCP / Precedence.
Bottom of Stack (S bit)	1 bit	`S = 1` indicates the bottom (innermost) label in a stack. `S = 0` indicates more labels follow.
Time to Live (TTL)	8 bits	Decremented at every MPLS hop to prevent routing loops. Copied from IP TTL at ingress.

Reserved Label Values (0 – 15)¶

RFC 3032 reserves the first 16 label values for special control plane functions:

Label 0 (Explicit Null for IPv4): Instructs the egress router to preserve TC/QoS bits while popping the label.
Label 2 (Explicit Null for IPv6): Preserves IPv6 Traffic Class / QoS bits on egress.
Label 3 (Implicit Null): Instructs the upstream (penultimate) router to pop the label (Penultimate Hop Popping — PHP), delivering an unlabelled IP packet to the egress router.

MPLS Label Operations: Push, Swap, Pop¶

graph LR
    IngressPE["Ingress PE Router<br/>(PUSH Label 16002)"] <===>|Swapped Path| CoreP["Core P Router<br/>(SWAP 16002 -> 16003)"]
    CoreP <===>|PHP Popped Path| EgressPE["Egress PE Router<br/>(POP Inner Service Label)"]

    classDef pe fill:#1b5e20,stroke:#81c784,color:#ffffff,stroke-width:2px,font-weight:bold;
    classDef p fill:#0d47a1,stroke:#64b5f6,color:#ffffff,stroke-width:2px,font-weight:bold;

    class IngressPE,EgressPE pe; class CoreP p;

PUSH (Imposition): Performed by the Ingress PE. Inserts one or more MPLS shim headers onto an unlabelled IP packet entering the MPLS domain.
SWAP: Performed by intermediate P Core Routers. Replaces the incoming top MPLS label with a new outgoing label learned from LDP/SR.
POP (Disposition): Performed by the Penultimate Hop (PHP) or Egress PE. Removes the top MPLS label header.

Control Plane vs Data Plane Tables (LIB vs LFIB)¶

graph TD
    ControlPlane["Control Plane<br/>LDP / BGP / Segment Routing"] --->|Learns Labels| LIB["LIB (Label Information Base)<br/>All Advertised Binding Pairs"]
    RIB["IP RIB (Routing Table)"] --->|Selects Best Path| FIB["IP FIB (Forwarding Table)"]
    LIB --->|Installs Best Label| LFIB["LFIB (Label Forwarding Information Base)<br/>Hardware ASIC TCAM Table"]
    FIB ---> LFIB

    classDef control fill:#e65100,stroke:#ffb74d,color:#ffffff,stroke-width:2px,font-weight:bold;
    classDef data fill:#2e7d32,stroke:#a5d6a7,color:#ffffff,stroke-width:2px,font-weight:bold;
    class ControlPlane,LIB control; class RIB,FIB,LFIB data;

LIB (Label Information Base): Control-plane database storing all label bindings received from all LDP peers.
LFIB (Label Forwarding Information Base): Hardware ASIC data-plane table containing only the best incoming label → action (Push/Swap/Pop) → outgoing label → next-hop interface mappings.