What does split horizon prevent?
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Split horizon is a technique used in computer networking to prevent routing loops in distance vector routing protocols. Routing loops occur when a packet is continually routed between two or more routers because none of the routers has up-to-date information about the network topology. This can lead to excessive network traffic and degraded network performance.
In a distance vector routing protocol, each router maintains a routing table that contains information about the entire network topology. When a router receives a packet, it consults its routing table to determine the best path to the packet's destination. The router then forwards the packet to the next hop on that path.
Split horizon is a mechanism that prevents a router from advertising a route back to the same interface from which the route was learned. For example, if Router A learns a route to a network through its interface Fa0/0, it will not advertise that route back out of Fa0/0 to Router B. This prevents a routing loop between Router A and Router B.
Split horizon can be further enhanced with a technique called route poisoning. Route poisoning involves setting the metric for a route to infinity (usually 16 hops) and advertising the poisoned route back to the router that originally advertised it. This notifies the router that the route is no longer available and prevents it from forwarding packets to the non-existent network.
To sum up, Split horizon prevents routing loops in distance vector routing protocols by preventing a router from advertising a route back to the same interface from which the route was learned. This is achieved by not advertising the route to the interface where it was learned.