After configuring ip summary-address eigrp 100 10.0.0.0 255.255.248.0 on R1's interface toward the branch routers, what is the immediate effect on EIGRP advertisements sent out that interface?

Configuring ip summary-address eigrp on an interface creates a summary route that EIGRP advertises out that specific interface. The primary effect is route summarization: R1 will advertise the single summary route (10.0.0.0/21 in this case) to its neighbors on that interface. Crucially, EIGRP automatically suppresses the advertisement of all more-specific routes that fall within the summary range on that same interface. This reduces routing table size and improves network stability by minimizing the impact of topology changes. R1 does not remove the more-specific routes from its local routing table; they remain for internal forwarding. The summary is not converted into dynamic EIGRP-learned routes, nor does R1 continue to advertise both the summary and all more-specific routes concurrently out the same interface, as that would defeat the purpose of summarization.