EIGRP - Router ID and loop prevention
The Router ID is used to uniquely identify a router within the topology of a dynamic routing protocol. When it comes to EIGRP, the router ID originally had an additional function: to prevent routing loops
Specifically, the EIGRP Router ID was primarily used to prevent routing loops in EIGRP environments utilizing redistribution. The Router ID identifies the originating router for external routes injected into the EIGRP domain.
Each external route was tagged with the Router ID of the router that redistributed it into EIGRP. If an external route is received with the same RID as the local router, the route is discarded. This feature is designed to reduce the risk of routing loops in networks where route redistribution occurs on multiple routers.
Now initially, the EIGRP Router ID was not advertised with internal routes. However, in recent IOS releases, the EIGRP Router ID is now advertised with internal routes as well. Consequently, each route advertised in EIGRP, whether internal or external, includes the Router ID of the router that injected it. The logic of using the Router ID remains unchanged—a router will discard any received route that carries its own Router ID.
Links:
https://forum.networklessons.com/t/eigrp-router-id/1246/17?u=lagapidis