Understanding MPLS Forwarding Mechanism
MPLS (Multiprotocol Label Switching) is a high-performance forwarding mechanism that operates on two planes:
1️⃣Control Plane:
EstablishesLabel Switched Paths (LSPs)using protocols likeLDP (Label Distribution Protocol),RSVP-TE (Resource Reservation Protocol - Traffic Engineering), orBGP-LU (BGP Label Unicast).
Assignslabelsto packets based on routing policies.
2️⃣Data Plane (Forwarding Plane):
Uses aLabel Forwarding Information Base (LFIB)instead of traditional IP routing.
Packets areforwarded 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 aTunnel IDto applications that utilize tunnels (e.g.,VPNs, MPLS-TE, or L2/L3 services).
This ID helpsdifferentiate 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 routersdo NOT use the FIB (Forwarding Information Base)for forwarding once labels are assigned.
Instead, they useLFIB (Label Forwarding Information Base)to switch packets based on labels.
❌B. If the tunnel ID is 0x0, the MPLS forwarding process starts.
Incorrect:If theTunnel ID is 0x0, MPLSdoes NOT start; it means no label is assigned, and traditionalIP forwardingoccurs.
❌D. If the tunnel ID is not 0x0, the normal IP forwarding process starts.
Incorrect:If the Tunnel ID isnot0x0, MPLSforwards traffic using label switching, not IP forwarding.
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