MPLS Fundamentals and Architecture
What this domain tests
MPLS Fundamentals and Architecture tests the label-forwarding model: how the control plane builds bindings, how the data plane uses them, and where the shim header sits. You should distinguish an LER from an LSR and a LIB from an LFIB.
Core rules
- A label entry is 32 bits: 20-bit label, 3-bit EXP, 1-bit bottom-of-stack, 8-bit TTL. The stack sits between the Layer 2 header and the MPLS payload.
- Labels are locally significant. The incoming interface plus incoming label selects an LFIB entry, which supplies the outgoing label, next hop, and outgoing interface.
- The LIB holds label bindings learned from protocols such as LDP or RSVP-TE. The LFIB holds the binding selected for forwarding.
- An LER pushes or pops labels at the edge; an LSR swaps labels in the core. Reserved labels 0–15 include implicit-null and IPv4 explicit-null.
Worked example
CE-A sends a packet to 192.0.2.9. PE1's LFIB maps that prefix to outgoing label 100 and next hop 10.0.0.2. PE1 pushes 100 toward P1. P1 receives label 100, swaps it to 200, and forwards to PE2. PE2 either pops the label or, if P1 advertised implicit-null, receives a plain IP packet. Each transit router consults only its incoming label; it does not look up 192.0.2.9.
Traps
Do not treat labels as globally unique, confuse the RIB with the LFIB, or place the shim inside the IP header. Implicit-null is advertised but not sent on the wire.