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MPLS: Forwarding Plane

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Forwarding Plane

What this domain tests

The Forwarding Plane domain tests data-plane mechanics after a label binding exists: how an incoming label selects an NHLFE, how the next hop is resolved, and how ECMP and adjacency affect forwarding. It is less about signaling and more about what the router does with a labeled packet.

Core rules

  • The LFIB is the forwarding table for labeled packets. A lookup returns an NHLFE containing the outgoing label operation, next hop, and outgoing interface.
  • The incoming label is meaningful only in the context of the interface on which it arrived. The same label value can map to different NHLFEs on different interfaces.
  • The next hop must be resolved through an adjacency. If the adjacency is down or unresolved, the LFIB entry cannot forward traffic even if the label binding exists.
  • ECMP can install multiple NHLFEs for the same incoming label. Hashing typically uses the IP header or, with entropy labels, label-stack fields.
  • The forwarding plane does not run LDP or BGP. It uses the installed result; control-plane convergence and data-plane convergence are separate.

Worked example

P1 receives a packet with top label 400 on interface Gi0/1. Its LFIB has two entries for (Gi0/1, 400): one to next hop 10.0.0.2 with outgoing label 500, and one to 10.0.0.6 with outgoing label 600. Both adjacencies are up, so P1 hashes the packet's flow and chooses one NHLFE. If the adjacency to 10.0.0.2 fails, that entry is removed or marked unusable, and traffic uses the remaining path. The destination IP is not used to choose the label entry.

Traps

Do not confuse the LFIB with the IP RIB. Do not assume a label binding proves forwarding works; the next-hop adjacency must be valid. Do not ignore ECMP hashing behavior or asymmetric paths.