Onnowpurbo: /* Example */

2018-12-22T09:07:40Z

Example

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	In this example, network node ''A'' routes packets to node ''B'' in order to reach node ''C''. The links between the nodes are distinct point-to-point links.		In this example, network node ''A'' routes packets to node ''B'' in order to reach node ''C''. The links between the nodes are distinct point-to-point links.

	[[~~Image~~:A-B-C.~~svg~~\|~~300px~~\|~~center~~\|]]		[[File:Router-a-b-c.png\|center\|400px\|thumb]]

	According to the split-horizon rule, node ''A'' does not advertise its route for ''C'' (namely ''A'' to ''B'' to ''C'') back to ''B''. On the surface, this seems redundant since ''B'' will never route via node ''A'' because the route costs more than the direct route from ''B'' to ''C''. However, if the link between ''B'' and ''C'' goes down, and ''B'' had received a route from ''A'' to ''C'', ''B'' could end up using that route via ''A''. ''A'' would send the packet right back to ''B'', creating a loop. This is the [[Count-to-infinity problem\|Count to Infinity Problem]]. With the split-horizon rule in place, this particular loop scenario cannot happen, improving [[Convergence (routing)\|convergence time]] in complex, highly-redundant environments.		According to the split-horizon rule, node ''A'' does not advertise its route for ''C'' (namely ''A'' to ''B'' to ''C'') back to ''B''. On the surface, this seems redundant since ''B'' will never route via node ''A'' because the route costs more than the direct route from ''B'' to ''C''. However, if the link between ''B'' and ''C'' goes down, and ''B'' had received a route from ''A'' to ''C'', ''B'' could end up using that route via ''A''. ''A'' would send the packet right back to ''B'', creating a loop. This is the [[Count-to-infinity problem\|Count to Infinity Problem]]. With the split-horizon rule in place, this particular loop scenario cannot happen, improving [[Convergence (routing)\|convergence time]] in complex, highly-redundant environments.

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{{Redirect|Split horizon|the Domain Name System feature|Split-horizon DNS}}
In [[computer networking]], '''split-horizon route advertisement''' is a method of preventing [[Routing loop problem|routing loop]]s in [[distance-vector routing protocol]]s by prohibiting a router from advertising a route back onto the interface from which it was learned.

The concept was suggested in 1974 by [[Torsten Cegrell]], and originally implemented in the [[Arpanet]] inspired Swedish network TIDAS.<ref>[http://ieeexplore.ieee.org/abstract/document/1092864/?reload=true A Routing Procedure for the TIDAS Message-Switching Network], IEEE Transactions on communication 1975</ref><ref>[https://www.internetmuseum.se/wordpress/wp-content/uploads/IMG_1407.jpg Letter from Torsten Cegrell to professor Leonard Kleinroch], 1974-08-19</ref><ref>[https://www.internetmuseum.se/tidslinjen/torsten-cegrell-svensken-som-lagade-internet/ Torsten Cegrell - the swede who "fixed" the Internet], Internetmuseum.se, access date 2017-11-09</ref>

==Example==
In this example, network node ''A'' routes packets to node ''B'' in order to reach node ''C''. The links between the nodes are distinct point-to-point links.

[[Image:A-B-C.svg|300px|center|]]

According to the split-horizon rule, node ''A'' does not advertise its route for ''C'' (namely ''A'' to ''B'' to ''C'') back to ''B''. On the surface, this seems redundant since ''B'' will never route via node ''A'' because the route costs more than the direct route from ''B'' to ''C''. However, if the link between ''B'' and ''C'' goes down, and ''B'' had received a route from ''A'' to ''C'', ''B'' could end up using that route via ''A''. ''A'' would send the packet right back to ''B'', creating a loop. This is the [[Count-to-infinity problem|Count to Infinity Problem]]. With the split-horizon rule in place, this particular loop scenario cannot happen, improving [[Convergence (routing)|convergence time]] in complex, highly-redundant environments.

''Split-horizon routing with poison reverse''<ref>[https://books.google.com/books?id=CN5MWZddE0QC&lpg=PP1&dq=Distance-vector%20routing%20decnet&pg=PA114#v=onepage&q=&f=false IP routing protocols By Uyless D. Black]</ref> is a variant of split-horizon route advertising in which a router actively advertises routes as unreachable over the interface over which they were learned by setting the route metric to infinite (16 for [[Routing information protocol|RIP]]). The effect of such an announcement is to immediately remove most looping routes before they can propagate through the network.

The main disadvantage of poison reverse is that it can significantly increase the size of routing announcements in certain fairly common network topologies, but it allows for the improvement of the overall efficiency of the network in case of faults.
Split horizon states that if a neighboring router sends a route to a router, the receiving router will not propagate this route back to the advertising router on the same interface.

With route poisoning, when a router detects that one of its connected routes has failed, the router will poison the route by assigning an infinite metric to it and advertising it to neighbors. When a router advertises a poisoned route to its neighbors, its neighbors break the rule of split horizon and send back to the originator the same poisoned route, called a poison reverse.
In order to give the router enough time to propagate the poisoned route and to ensure that no routing loops occur while propagation occurs, the routers implement a hold-down mechanism.

==Implementations==
The split-horizon method is effective and simple to implement, and is therefore used by most distance-vector protocols. It is notably used by:
* [[Routing information protocol|RIP]]
* [[Interior Gateway Routing Protocol|IGRP]]
* [[Enhanced Interior Gateway Routing Protocol|EIGRP]]
* [[Virtual Private LAN Service|VPLS]], which uses the split horizon technique to avoid loops in the forwarding plane
* [[Babel (protocol)|Babel]], which applies split-horizon to wired links only

==See also==
* [[Split-horizon DNS]], a similar concept in [[DNS]] performing selective publishing of information
* [[Route poisoning]]

==References==
{{reflist|2}}

==External links==
* "Split Horizon" and "Split Horizon with Poison Reverse" in [http://www.microsoft.com/technet/prodtechnol/windows2000serv/reskit/intwork/inae_ips_vzbs.mspx?mfr=true Convergence in RIP Internetworks], from Microsoft TechNet.

[[Category:Routing protocols]]

Split Horizon - Revision history

Onnowpurbo: /* Example */

Onnowpurbo: Created page with "{{Redirect|Split horizon|the Domain Name System feature|Split-horizon DNS}} In computer networking, '''split-horizon route advertisement''' is a method of preventing Rou..."