Route tags are the correct loop-prevention mechanism when connecting another routing domain or redistributed network behind R3 into an EIGRP network. Redistribution can create route feedback: a route learned from one domain is redistributed into another, then later redistributed back into the original domain as if it were new information. That can produce routing loops or suboptimal path selection, especially in multipoint redistribution designs. Cisco route-map based redistribution commonly uses route tags to mark the origin of redistributed routes. Other redistribution points can then match those tags and prevent the same routes from being reintroduced into the source domain. Split horizon prevents advertisements back out the interface on which a route was learned, but it is not sufficient for multipoint redistribution loop prevention.
Summarization reduces route detail and query scope but does not identify route origin. The OSPF down bit is used in MPLS VPN/OSPF PE-CE scenarios, not general EIGRP redistribution. Therefore, the design should use route tags. Reference topics: EIGRP redistribution, route tagging, routing-loop prevention, route maps, redistribution policy.