LSP ping and P2MP LSP ping operate over a network using unnumbered links without any changes. LSP trace, P2MP LSP trace and LDP treetrace are modified such that the unnumbered interface is correctly encoded in the downstream mapping (DSMAP/DDMAP) TLV.
In a RSVP P2P or P2MP LSP, the upstream LSR encodes the downstream router ID in the Downstream IP Address field and the local unnumbered interface index value in the Downstream Interface Address field of the DSMAP/DDMAP TLV as per RFC 4379. Both values are taken from the TE database.
In a LDP unicast FEC or mLDP P2MP FEC, the interface index assigned by the peer LSR is not readily available to the LDP control plane. In this case, the alternative method described in RFC 4379 is used. The upstream LSR sets the Address Type to IPv4 Unnumbered, the Downstream IP Address to a value of 127.0.0.1, and the interface index is set to 0. If an LSR receives an echo-request packet with this encoding in the DSMAP/DDMAP TLV, it will bypass interface verification but continue with label validation.