RIPv2 Split Horizon verification and Route Poisioning.

Configuration Router1 on top left:

Before Split Horizon is enabled on an Interface.  
debug ip rip (partial output)

RIP: received v2 update from 192.168.2.2 on GigabitEthernet0/1

192.168.1.0/24 via 0.0.0.0 in 2 hops

192.168.2.0/24 via 0.0.0.0 in 1 hops

192.168.3.0/24 via 0.0.0.0 in 1 hops

RIP: received v2 update from 192.168.2.2 on GigabitEthernet0/1

192.168.1.0/24 via 0.0.0.0 in 2 hops

192.168.2.0/24 via 0.0.0.0 in 1 hops

192.168.3.0/24 via 0.0.0.0 in 1 hops

Split Horizon is enabled:

Router(config)#int g0/1

Router(config-if)#ip split-horizon

Router(config-if)#int g0/0

Router(config-if)#ip split-horizon

After Split Horizon is enabled:

debug ip rip (partial output)

RIP: received v2 update from 192.168.2.2 on GigabitEthernet0/1

192.168.3.0/24 via 0.0.0.0 in 1 hops

RIP: received v2 update from 192.168.2.2 on GigabitEthernet0/1

192.168.3.0/24 via 0.0.0.0 in 1 hops

Explanation:

Split Horizon when enabled will not send updates through an

Interface from which it has learned those subnets.

IN above debug output only 192.168.3.0 is received on the

Router on the left.On the Router on the Right only 192.168.1.0

will be received .

3.0 and 1.0 subnets have no interfaces with split horizon enabled on them.

Split Horizon has an impact on Routing updates. 

224.0.0.9 is the RIPv2 Multicast.

Similar behavior is seen on Router2 on the right.

Before the Split:

RIP: received v2 update from 192.168.2.1 on GigabitEthernet0/1

192.168.1.0/24 via 0.0.0.0 in 1 hops

192.168.2.0/24 via 0.0.0.0 in 1 hops

192.168.3.0/24 via 0.0.0.0 in 2 hops

After the Split:

RIP: received v2 update from 192.168.2.1 on GigabitEthernet0/1

192.168.1.0/24 via 0.0.0.0 in 1 hops

Route Poisoning:

A downed route/subnet is given a metric of 16,infinity in RIP
RIP hop count max's out at 15. If a 16 Metric is seen in
the debug output than that indicates route poisoning,
that route is down.

RIP: sending v2 update to 224.0.0.9 via GigabitEthernet0/0 (192.168.1.2)

RIP: build update entries

192.168.2.0/24 via 0.0.0.0, metric 1, tag 0

192.168.3.0/24 via 0.0.0.0, metric 16, tag 0