The address of a configured loopback interface, other than the router ID, can be used as the destination of an RSVP LSP. For a constrained-path LSP, CSPF searches for the best path that matches the constraints across all areas or levels of the IGP where this address is reachable. If the address is the router ID of the destination node, then CSPF selects the best path across all areas or levels of the IGP for that router ID, regardless of which area or level the router ID is reachable for as an interface.
The address of a loopback interface other than the router ID can also be configured as a hop in the LSP path hop definition. If the hop is ‟strict” and corresponds to the router ID of the node, the CSPF path may use any TE-enabled link to the downstream node based on best cost. If the hop is ‟strict” and does not correspond to the router ID of the node, CSPF will fail.