How does split-horizon in eigrp really works in my topology?

[Abdullah Net]

Hey there,

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 14.0.0.0/30, 1 successors, FD is 1310720, serno 75

        via 13.0.0.2 (1310720/1308160), FastEthernet0/1

        via 12.0.0.2 (1843200/1331200), Serial0/0/1

        via 10.0.0.2 (2195456/1305600), Serial0/1/0

P 17.0.0.0/30, 1 successors, FD is 645120, serno 7

        via 13.0.0.2 (645120/514560), FastEthernet0/1

        via 12.0.0.2 (1052160/540160), Serial0/0/1

        via 10.0.0.2 (2198016/1308160), Serial0/1/0

P 15.0.0.0/30, 1 successors, FD is 2565120, serno 80

        via 11.0.0.2 (2565120/2562560), FastEthernet0/0

        via 13.0.0.2 (2567680/2565120), FastEthernet0/1

        via 12.0.0.2 (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 13.0.0.2 (773120/642560), FastEthernet0/1

        via 12.0.0.2 (985600/409600), Serial0/0/1

        via 10.0.0.2 (2323456/1433600), Serial0/1/0

P 16.0.0.0/30, 1 successors, FD is 670720, serno 76

        via 13.0.0.2 (670720/540160), FastEthernet0/1

        via 12.0.0.2 (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 15.0.0.0 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 15.0.0.0 is the one that got all four routes. Why not network 14.0.0.0 for example? In other words, how does split-horizon works in this topology. (step by step would be appreciated )

Thanks,

Abdul,

[Jon Marshall]

Abdul

Can you post a "sh ip eigrp topology all-links" from R3 please.

Jon

[vijayvitthalayachit96]

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.