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Split Horizon and Poisoned Reverse, methods of computer network technology and are used in certain routing protocols to prevent the exchange of information between routers routing loops.

Split horizon methods are used in the protocols RIP, EIGRP, IGRP and VPLS.

Operation

When split horizon in the routing table, the number of hops and the intermediate routers to be saved to a destination network and the information about the router from which information has been received.

The router does not send routing updates on the interface from which he has learned the networks.

Example

Router C "knows" of Router B that the network N0 can be reached via Router A.

Normal state:

Network N0 → Router A Router B ↔ ↔ Router C Not reach Router A: failure of a router

Network N0 → Router A Router B → ← XX ↔ Router C Without split horizon

• Router B learns the next update from Router C to Router A is (originally from C to B and from B to A) to reach over him in 2 hops.
• Router B adjusts the routing table and increases the hop count by 1
• During update router learns C from B that router A is now accessible with 3 hops via router B.
• During update Router B receives from C, the information that Router A is now accessible with 4 hops.

Thus, a routing loop has been created up to 16 hops, which counts the maximum possible number of positions with RIP. Only then the route is marked on Router A as unreachable. Since updates are exchanged every 30 seconds, it can be 15 times 30 seconds, or take a good seven minutes, until the failure of a router detected and the connection is marked as inaccessible to him.

With split horizon

• Router C sends any updates from the network to the interface from which he has learned the network, and thus avoids routing loops.

Poisoned Reverse

When poisoned reverse ( with blocked return route ) all over this interface learned / received routes will be marked as "unavailable " and sent back by the number of hops is set directly on 16, so this route is not taken into account by other routers, as each other inevitably Hop "infinite" (16 ) would mean.

• Routing