RIP: do not split it then… and unleach the power of the “network” command

In RIP domain routers R1 and R2 exchange information about loopback 0 and 1 networks

Router RIP command “network” 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 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 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 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 is considered behind the nexthop not attached to it

Instead the subnet 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 (

Let’s confirm this by first adding a new directly attached network 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 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


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