Editing Holddown

{{About|the network routing heuristic}}

'''Holddown''' works by having each router start a timer when it first receive information about a network that is unreachable. Until the timer expires, the router will discard any subsequent route messages that indicate the route is actually reachable. It can resolve situations where multiple routers are connected indirectly. There are realistic scenarios where [[split horizon]] and split horizon with poisoned reverse are ineffective.<ref>{{cite web |url=http://www.tcpipguide.com/free/t_RIPSpecialFeaturesForResolvingRIPAlgorithmProblems-4.htm |title=RIP Special Features For Resolving RIP Algorithm Problems |date=2005-09-20 |accessdate=2012-01-09 |publisher=The TCP/IP Guide }}</ref>

In other words, a holddown prevents a [[Router (computing)|router]] from receiving route updates until the network appears to be stable—until either an [[Interface (computer science)|interface]] stops changing state (flapping) or a better route is learned.

Holddowns are typically implemented with [[timer]]s. If the router detects that a network is unreachable, the timer is started. The router will then wait a preset number of seconds until the network stabilizes. When the timer expires, the router will begin receiving its routing updates from other routers. For example, in [[Routing Information Protocol|RIP]], the default holddown timer is set to 180 seconds.

==References==
{{Reflist}}<!--added above categories/infobox footers by script-assisted edit-->

[[Category:Routing protocols]]