Understanding MPLS Forwarding Mechanism
MPLS (Multiprotocol Label Switching) is a high-performance forwarding mechanism that operates on two planes:
1️⃣ Control Plane:
Establishes Label Switched Paths (LSPs) using protocols like LDP (Label Distribution Protocol), RSVP-TE (Resource Reservation Protocol - Traffic Engineering), or BGP-LU (BGP Label Unicast).
Assigns labels to packets based on routing policies.
2️⃣ Data Plane (Forwarding Plane):
Uses a Label Forwarding Information Base (LFIB) instead of traditional IP routing.
Packets are forwarded based on labels, not IP addresses.
Analysis of the Answer Choices
✅ C. The system automatically assigns an ID to the upper-layer application that uses a tunnel. This ID is also called the tunnel ID.
Correct: MPLS assigns a Tunnel ID to applications that utilize tunnels (e.g., VPNs, MPLS-TE, or L2/L3 services).
This ID helps differentiate between various tunnel applications, ensuring traffic follows the correct LSP.
❌ A. After an IP packet enters an MPLS domain, the MPLS device forwards the packet based on FIB table queries.
Incorrect: MPLS routers do NOT use the FIB (Forwarding Information Base) for forwarding once labels are assigned.
Instead, they use LFIB (Label Forwarding Information Base) to switch packets based on labels.
❌ B. If the tunnel ID is 0x0, the MPLS forwarding process starts.
Incorrect: If the Tunnel ID is 0x0, MPLS does NOT start; it means no label is assigned, and traditional IP forwarding occurs.
❌ D. If the tunnel ID is not 0x0, the normal IP forwarding process starts.
Incorrect: If the Tunnel ID is not 0x0, MPLS forwards traffic using label switching, not IP forwarding.
