If an LSR is the ingress router for a specific IP prefix, LDP programs a PUSH operation for the prefix in the IOM. This creates an LSP ID to the Next Hop Label Forwarding Entry (NHLFE) mapping (LTN mapping) and an LDP tunnel entry in the forwarding plane. LDP also informs the Tunnel Table Manager (TTM) about this tunnel. Both the LSP ID to NHLFE (LTN) entry and the tunnel entry have an NHLFE for the label mapping that the LSR received from each of its next-hop peers.
If the LSR is to function as a transit router for a specific IP prefix, LDP programs a SWAP operation for the prefix in the IOM. This involves creating an Incoming Label Map (ILM) entry in the forwarding plane. The ILM entry may need to map an incoming label to multiple NHLFEs.
If an LSR is an egress router for a specific IP prefix, LDP programs a POP entry in the IOM. This too results in an ILM entry being created in the forwarding plane, but with no NHLFEs.
When unlabeled packets arrive at the ingress LER, the forwarding plane consults the LTN entry and uses a hashing algorithm to map the packet to one of the NHLFEs (PUSH label) and forward the packet to the corresponding next-hop peer. For a labeled packet arriving at a transit or egress LSR, the forwarding plane consults the ILM entry and either uses a hashing algorithm to map it to one of the NHLFEs if they exist (SWAP label) or routes the packet if there are no NHLFEs (POP label).