If an LSR is the ingress for a specific IP prefix, LDP programs a 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 will also inform the Tunnel Table Manager (TTM) of this tunnel. Both the LTN entry and the tunnel entry will 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 specified IP prefix, LDP will program a swap operation for the prefix in the forwarding engine. This results in the creation of an Incoming Label Map (ILM) entry in the forwarding plane. The ILM entry will have to map an incoming label to possibly multiple NHLFEs. If the LSR is an egress for a specific IP prefix, LDP programs a POP entry in the forwarding engine. Programming a POP entry results in an ILM entry in the forwarding plane, but with no NHLFEs.
When unlabeled packets arrive at the ingress LER, the forwarding plane will consult the LTN entry and will use 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 will consult 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 will not be activated unless there is a matching route in system route table that also matches the user configured static FEC next-hop.