In RIP domain routers R1 and R2 exchange information about loopback 0 and 1 networks
Router RIP command “network 0.0.0.0” hints routers to add all ip enabled interfaces to the update list and include their “subnets” into this update
I add a static route to network 88.88.88.88/32 using the outgoing interface (R1 lo0) instead of the next hop or intermediate address
The static route is valid in the default routing table
And also present in rip database as “connected”
The new added route is sent to all RIP enabled interfaces except Lo0
The update is sent to all interfaces except the one the route was configured with
The router already knows the route is behind lo0
There’s then no need to re advertise it back via this interface
Let’s check deactivate the split horizon on this lo0 interface and run another debug ip rip
Now the route 88.88.88.88/32 is sent to all the interfaces
Including the interface the route was configured with (or routing information received, hinted, from it)
Now I add another route but that points to a next hop address
The route to 99.99.99.99/32 is valid and in the default routing table
But the route is no more in the rip routing table
And the route is not included in any routing updates
To build its update list, networks that are in this list, the router checks its rip routing table
All the information is sent over an interface (a RIP enabled) except the information received on this interface by default using the split-horizon rule…
The subnet 99.99.99.99 is considered behind the nexthop 1.2.1.254 not attached to it
Instead the subnet 88.88.88.88 is considered as directly connected because no next hop information has been provided
The network command has enabled rip udpate sending and receiving on the lo0 interface but added into the update list all the directly connected networks attached to this interface and not only the primary address subnet of this interface (1.1.1.1/32)
Let’s confirm this by first adding a new directly attached network 77.77.77.77/32 to lo0
The new route is added the update list and sent over all the rip enabled interfaces
Let’s change the rip router configuration network command to include only the physical and loopback interfaces
The new configuration
Nothing has changed, the router still advertises all the attached networks through the rip enabled interfaces
We use redistribution to get the 99.99.99.99/32 route into rip domain
In the rip database
The route is marked as “redistributed” and sent to all interfaces except lo1
Because of split horizon rule that is preventing udpates back on the interface they were received on by default