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+.