What is Split Horizon?

Split horizon is a method of preventing a routing loop in a network.

Meaning whenever loop formation occurs in the network Split horizon will prevent that. Now how it will prevent loop formation, the basic principle behind the split-horizon is that the routing source of any packet will never send back to its source from which it is generated.

Split Horizon in RIP:

Distance vector protocols (RIP and EIGRP) are more susceptible to routing loops. Routing loops occur when a packet is continually routed through the same routers over and over in repetition and forms loop.

Split horizon is one of the methods used by distance vector routing protocols to avoid routing loops. This is achieved as – a router will not advertise a route back onto the interface from which it was learned. Split horizon is enabled on interfaces by default.

To consider what could happen without the split-horizon mechanism, take a look at the following example:

We let us assume that we have a network of three routers (R1, R2, and R3). All routers are running RIP Protocol, i.e. a distance vector protocol. R3 is directly connected to the 10.0.0.0/24 network and advertises that network using RIP to R2. R2 receives the routing update from R3, places the route in its routing table and informs Router R1 about the 10.0.0.0/24 network.

Now because the split-horizon mechanism is enabled by default on all interfaces, R1 will not advertise to R2 that it has the route to 10.0.0.0/24 network in its routing table. And here split horizon prevents the formation of the Routing loop in the network.

What if Split Horizon will not present:

Now consider what would happen if the split-horizon mechanism didn’t prevent Router R1 to advertise the route back to Router R2. Router R1 would advertise to Router R2 that it has a route to reach the 10.0.0.0/24 network. Let’s say that the link between the Router R2 and Router R3 fails. Since Router R2 received a route to that network from Router R1, it will send all packets destined for the 10.0.0.0/24 network to router R1. But Router R1 thinks that Router R2 has a route to reach that network (it doesn’t know that the link between Router R2 and Router R3 failed) and sends the packets back to Router R2, thereby creating a routing loop.