The LDP over RSVP capability set includes the ability to stitch LDP-over-RSVP tunnels at internal (non-ABR) OSPF and IS-IS routers.
In Figure 1, assume that the user wants to use LDP over RSVP between router A and destination ‟Dest”. The first thing that happens is that either OSPF or IS-IS performs an SPF calculation resulting in an SPF tree. This tree specifies the lowest possible cost to the destination. In the example shown, the destination ‟Dest” is reachable at the lowest cost through router X. The SPF tree has the following path: A>C>E>G>X.
Using this SPF tree, router A searches for the endpoint that is closest (farthest/highest cost from the origin) to ‟Dest” that is eligible. Assuming that all LSPs in the above diagram are eligible, LSP endpoint G is selected as it terminates on router G while other LSPs only reach routers C and E, respectively.
IGP and LSP metrics associated with the various LSP are ignores; only tunnel endpoint matters to IGP. The endpoint that terminates closest to ‟Dest” (highest IGP path cost) is selected for further selection of the LDP over RSVP tunnels to that endpoint. The explicit path the tunnel takes may not match the IGP path that the SPF computes.
If router A and G have an additional LSP terminating on router G, there would now be two tunnels both terminating on the same router closest to the final destination. For IGP, it does not make any difference on the numbers of LDPs to G, only that there is at least one LSP to G. In this case, the LSP metric is considered by LDP when deciding which LSP to stitch for the LDP over RSVP connection.
The IGP only passes endpoint information to LDP. LDP looks up the tunnel table for all tunnels to that endpoint and picks up the one with the least tunnel metric. There may be many tunnels with the same least cost. LDP FEC prefixes with a subnet mask lower or equal than 32 is resolved over RSVP LSPs within an area.