Split horizon is a technique used to avoid routing loops.
With split horizon, when a route is learned on an interface, that route is not advertised back out on the same interface.
For example, RT2 receives an update about Network X with a metric of 1 from the neighboring RT1.
RT2 will not advertise Network X back to RT1 if split horizon is enabled.
If split horizon is disabled, however, RT2 will advertise Network X with a metric of 2 to RT1.
If Network X fails, RT1 will think that RT2 has a better way to get to X, so it will send the packet destined to Network X toward RT1, creating a black hole.
RT1#sh cdp neighbors Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge S - Switch, H - Host, I - IGMP, r - Repeater Device ID Local Intrfce Holdtme Capability Platform Port ID RT2 Ser 0/0 153 R 3620 Ser 0/0 RT1# RT1#sh ip int brief Interface IP-Address OK? Method Status Protocol Serial0/0 192.168.2.1 YES manual up up FastEthernet1/0 192.168.1.1 YES manual up up RT1# RT1#debug ip rip RIP protocol debugging is on RT1# RT1#conf t Enter configuration commands, one per line. End with CNTL/Z. RT1(config)#router rip RT1(config-router)#network 192.168.1.0 RT1(config-router)#network 192.168.2.0 RT1(config-router)#exit RT1(config)#end RT1# RT1#sh ip route Gateway of last resort is not set C 192.168.1.0/24 is directly connected, FastEthernet1/0 C 192.168.2.0/24 is directly connected, Serial0/0 RT1# 00:02:16: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:02:16: RIP: build update entries 00:02:16: network 192.168.1.0 metric 1 00:02:16: RIP: sending v1 update to 255.255.255.255 via FastEthernet1/0 (192.168.1.1) 00:02:16: RIP: build update entries 00:02:16: network 192.168.2.0 metric 1 00:02:46: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:02:46: RIP: build update entries 00:02:46: network 192.168.1.0 metric 1 00:02:46: RIP: sending v1 update to 255.255.255.255 via FastEthernet1/0 (192.168.1.1) 00:02:46: RIP: build update entries 00:02:46: network 192.168.2.0 metric 1 RT1# ================================================================================ RT2#sh ip int brief Interface IP-Address OK? Method Status Protocol Serial0/0 192.168.2.2 YES manual up up RT2# RT2#debug ip rip RIP protocol debugging is on RT2# RT2#conf t Enter configuration commands, one per line. End with CNTL/Z. RT2(config)#router rip RT2(config-router)#network 192.168.2.0 RT2(config-router)#exit RT2(config)#end RT2# 00:02:15: RIP: received v1 update from 192.168.2.1 on Serial0/0 00:02:15: 192.168.1.0 in 1 hops 00:02:24: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (192.168.2.2) 00:02:24: RIP: build update entries - suppressing null update 00:02:45: RIP: received v1 update from 192.168.2.1 on Serial0/0 00:02:45: 192.168.1.0 in 1 hops 00:02:53: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (192.168.2.2) 00:02:53: RIP: build update entries - suppressing null update/0 RT2# RT2#conf t Enter configuration commands, one per line. End with CNTL/Z. RT2(config)#int s0/0 RT2(config-if)#no ip split-horizon ? eigrp Enhanced Interior Gateway Routing Protocol (EIGRP) <cr> RT2(config-if)#no ip split-horizon RT2(config-if)#end RT2# 00:03:14: RIP: received v1 update from 192.168.2.1 on Serial0/0 00:03:14: 192.168.1.0 in 1 hops 00:03:20: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (192.168.2.2) 00:03:20: RIP: build update entries 00:03:20: network 192.168.1.0 metric 2 00:03:20: subnet 192.168.2.0 metric 1 RT2# ================================================================================ RT1# 00:03:16: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:03:16: RIP: build update entries 00:03:16: network 192.168.1.0 metric 1 00:03:16: RIP: sending v1 update to 255.255.255.255 via FastEthernet1/0 (192.168.1.1) 00:03:16: RIP: build update entries 00:03:16: network 192.168.2.0 metric 1 00:03:22: RIP: received v1 update from 192.168.2.2 on Serial0/0 00:03:22: 192.168.1.0 in 2 hops 00:03:22: 192.168.2.0 in 1 hops RT1#
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