Split Horizon
Split
horizon is a network routing concept primarily used in distance-vector
routing protocols like RIP, where a router prevents itself from advertising a
route back to the same router it learned that route from, effectively stopping
routing loops by avoiding the circulation of routing information in a circular
path within the network; essentially, it means a router will not send routing
updates about a route back through the interface it received that route on.
Key points about split horizon
Purpose: To prevent routing loops, a common issue in network routing where packets can get stuck circulating between routers endlessly.
How it works: When a router receives a route update from a neighbor, it will not advertise that same route back to the neighbor it received it from on the same interface.
Importance in distance-vector protocols: Split horizon is particularly important as they rely on periodic updates from neighboring routers, which could easily create loops without this mechanism.
Example scenario: Imagine Router A learns about a network from Router B. According to split horizon, Router A will not advertise that network back to Router B on the link connecting them directly.
Related concepts
Poison reverse: An extension of split horizon where a router not only doesn't advertise a route back to the source but also marks that route with an artificially high metric (considered "poisoned") to further discourage its usage.
BGP split horizon: While BGP is a path-vector protocol, the concept of split horizon is still applied where a router will not advertise a route back to the neighbor it learned from, although the implementation is slightly different than distance-vector protocols.
This is covered in Network+.