Holddown
Encyclopedia
Holddown timer works by having each router start a timer when they 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 in fact reachable. It can solve the case where multiple routers are connected indirectly. The classic example would be three routers configured in a triangle. In this situation, Split horizon
and split horizon with poisoned reverse can do nothing.
In other words, a holddown keeps a router from receiving route updates until the network appears to be stable—until either an interface
stops changing state (flapping) or a better route is learned.
Holddowns are usually implemented with timer
s. If the router detects a network unreachable, the timer is started. The router will then wait a preset number of seconds until the network is stable. When the timer expires, the router will then receive its routing updates from other routers. For example in RIP
the default holddown timer is set on 180 seconds.
Split horizon
In computer networking, split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned.-Example:...
and split horizon with poisoned reverse can do nothing.
In other words, a holddown keeps a router from receiving route updates until the network appears to be stable—until either an interface
Interface (computer science)
In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
stops changing state (flapping) or a better route is learned.
Holddowns are usually implemented with timer
Timer
A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...
s. If the router detects a network unreachable, the timer is started. The router will then wait a preset number of seconds until the network is stable. When the timer expires, the router will then receive its routing updates from other routers. For example in RIP
Routing Information Protocol
The Routing Information Protocol is a distance-vector routing protocol, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15....
the default holddown timer is set on 180 seconds.