When LDP tries to resolve a prefix received over a T-LDP session, it performs a lookup in the Routing Table Manager (RTM). This lookup returns the next hop to the destination PE and the advertising router (ABR or destination PE itself).
When searching for an LDP-over-RSVP tunnel, LDP selects the advertising routers with best route. If the advertising router matches the targeted LDP peer, LDP then performs a second lookup for the advertising router in the Tunnel Table Manager (TTM) which returns the user configured RSVP LSP with the best metric. If there are more than one configured LSP with the best metric, LDP selects the first available LSP.
If all user configured RSVP LSPs are down, then an LDP tunnel is selected if available.
If the user did not configure any LSPs under the T-LDP session, a lookup in TTM returns the first available RSVP LSP which terminates on the advertising router. If none are available, then an LDP tunnel is selected if available.