The user creates a targeted LDP (T-LDP) session to an ABR or the destination PE. This results in LDP hellos being sent between the two routers. These messages are sent unlabeled over the IGP path. Next, the user enables LDP tunneling on this T-LDP session and optionally specifies a list of LSP names to associate with this T-LDP session. By default, all RSVP LSPs which terminate on the T-LDP peer are candidates for LDP-over-RSVP tunnels. At this point in time, the LDP FECs resolving to RSVP LSPs are added into the Tunnel Table Manager as tunnel-in-tunnel type.
If LDP is running on regular interfaces also, the prefixes LDP learns are going to be distributed over both the T-LDP session as well as regular IGP interfaces. LDP FEC prefixes with a subnet mask lower or equal than 32 are resolved over RSVP LSPs. The policy controls which prefixes go over the T-LDP session, for example, only /32 prefixes, or a particular prefix range.
LDP-over-RSVP works with both OSPF and ISIS. These protocols include the advertising router when adding an entry to the RTM. LDP-over-RSVP tunnels can be used as shortcuts for BGP next-hop resolution.