Interior gateway protocols (IGPs) are configured under global configuration and then enabled on a specific interface. When this happens, the router will search for other routers on that specific interface that are also running the same protocol - this is accomplished by sending out and listening for “hello packets”. Whenever there is a match, they will form an adjacency with each other and exchange routing information.
Modern routing protocols use Multicast for the hello packets, which is more efficient than broadcasts used in older protocols.
Note
If we don’t include an interface as part of the routing protocol the router will not attempt to form adjacency on that interface. Additionally, it will not advertise the routes on the excluded interface to other routers on enabled interfaces. To allow an IP subnet to be included, we must use Passive Interfaces.