I have basic knowledge about what is split-horizon but not how it actually works.
Please refer to the topology below...
According to R1's "show ip eigrp topology all-links"
P 126.96.36.199/30, 1 successors, FD is 1310720, serno 75
via 188.8.131.52 (1310720/1308160), FastEthernet0/1
via 184.108.40.206 (1843200/1331200), Serial0/0/1
via 10.0.0.2 (2195456/1305600), Serial0/1/0
P 220.127.116.11/30, 1 successors, FD is 645120, serno 7
via 18.104.22.168 (645120/514560), FastEthernet0/1
via 22.214.171.124 (1052160/540160), Serial0/0/1
via 10.0.0.2 (2198016/1308160), Serial0/1/0
P 126.96.36.199/30, 1 successors, FD is 2565120, serno 80
via 188.8.131.52 (2565120/2562560), FastEthernet0/0
via 184.108.40.206 (2567680/2565120), FastEthernet0/1
via 220.127.116.11 (3100160/2588160), Serial0/0/1
via 10.0.0.2 (3100160/2588160), Serial0/1/0
P 172.30.20.0/30, 1 successors, FD is 773120, serno 77
via 18.104.22.168 (773120/642560), FastEthernet0/1
via 22.214.171.124 (985600/409600), Serial0/0/1
via 10.0.0.2 (2323456/1433600), Serial0/1/0
P 126.96.36.199/30, 1 successors, FD is 670720, serno 76
via 188.8.131.52 (670720/540160), FastEthernet0/1
via 184.108.40.206 (857600/281600), Serial0/0/1
via 10.0.0.2 (2221056/1331200), Serial0/1/0
As you can see, R1 has three routes to reach all networks except for network 220.127.116.11 which has four routes. So, R3 is not advertising routes to R1 except for its connected networks. I understand the reason that R1 doesn't have four routes to each network is due to the split-horizon effect.
I want to understand why network 18.104.22.168 is the one that got all four routes. Why not network 22.214.171.124 for example? In other words, how does split-horizon works in this topology. (step by step would be appreciated )
As per my knowledge, when an update is sent after adding a new network, the receiving router will also update that back in the same interface with higher metrics. For example, orinating router set scaled delay to 1 (just as a number), neighbor may set to 10 (depending on the metric formulae). By this originating router understands that for this route I am having lesser delay. So I discard this route.
Hope it helps.
Experts are welcome to correct me if I am wrong.