The first check for an LDP ID is through manual configuration. This would be if you've manually
configured the router and told it which interface to be used, or what loopback interface you want
used for MPLS. Part of manually specifying what interface a router should use is to configure the
IP address sitting on that interface as the Router ID. That's the first check: is there a configured
Router ID?
Second, if there is no configured LDP ID, MPLS will default to using the highest IP address on
any loopback interface the moment that LDP comes up. Third and last, in the event that we don't
have any loopbacks, as a last-ditch effort, the router will use the highest IP address on any of the
other interfaces.