Holddown 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. There are realistic scenarios where
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.
The concep ...
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
A timer is a specialized type of clock used for measuring specific time intervals.
Timers can be categorized into two main types.
The word "timer" is usually reserved for devices that counts down from a specified time interval, while devices th ...
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 is stable. When the timer expires, the router will then receive its routing updates from other routers. For example, in
RIP
Rest in peace (RIP), a phrase from the Latin (), is sometimes used in traditional Christian services and prayers, such as in the Catholic, Lutheran, Anglican, and Methodist denominations, to wish the soul of a decedent eternal rest and peace.
...
the default holddown timer is set to 180 seconds.
References
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Routing protocols