If an LSR is the ingress for a specific IP prefix, LDP programs push operation for the prefix in the forwarding engine. This creates an LSP ID to the Next Hop Label Forwarding Entry (NHLFE) (LTN) mapping and an LDP tunnel entry in the forwarding plane. LDP also informs the Tunnel Table Manager (TTM) of this tunnel. Both the LTN entry and the tunnel entry have a NHLFE for the label mapping that the LSR received from each of its next-hop peers.
If the LSR is to behave as a transit for a specific IP prefix, LDP programs a swap operation for the prefix in the forwarding engine. This involves creating an Incoming Label Map (ILM) entry in the forwarding plane. The ILM entry has to map an incoming label to possibly multiple NHLFEs. If an LSR is an egress for a specific IP prefix, LDP programs a POP entry in the forwarding engine. 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 labeled packets arriving at a transit or egress LSR, the forwarding plane consults the ILM entry and either use a hashing algorithm to map it to one of the NHLFEs if they exist (swap label) or simply route the packet if there are no NHLFEs (pop label).
Static FEC swap is not activated unless there is a matching route in system route table that also matches the user configured static FEC next-hop.