The implicit null label option enables an eLER to receive MPLS packets from the previous-hop LSR without the outer LSP label.
The implicit null label is signaled by the eLER to the previous-hop LSR during FEC signaling by the LDP control protocol. When the implicit null label is signaled to the LSR, it pops the outer label before sending the MPLS packet to the eLER; this is known as penultimate hop popping.
The implicit null label option can be enabled for all LDP FECs for which the router is the eLER by using the implicit-null-label command in the config>router>ldp context.
If the implicit null configuration is changed, LDP withdraws all the FECs and readvertises them using the new label value.