Holddown

Last updated April 07, 2019

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 and split horizon with poisoned reverse can do nothing.^[1]

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.

Router (computing) Device that forwards data packets between computer networks, creating an overlay internetwork

A router is a networking device that forwards data packets between computer networks. Routers perform the traffic directing functions on the Internet. Data sent through the internet, such as a web page or email, is in the form of data packets. A packet is typically forwarded from one router to another router through the networks that constitute an internetwork until it reaches its destination node.

Holddowns are usually implemented with timers. 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 the default holddown timer is set to 180 seconds.

Timer device that automatically times a process or event or activates an operation or another device at a preset time or times

A timer is a specialized type of clock used for measuring specific time intervals. Timers can be categorized into two main types. A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch, while a device which counts down from a specified time interval is more usually called a timer. A simple example of this type is an hourglass. Working method timers have two main groups: Hardware and Software timers.

The Routing Information Protocol ('RIP') is one of the oldest distance-vector routing protocols which employ 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 source to destination. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support.

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References

↑ "RIP Special Features For Resolving RIP Algorithm Problems". The TCP/IP Guide. 2005-09-20. Retrieved 2012-01-09.

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[1] "RIP Special Features For Resolving RIP Algorithm Problems". The TCP/IP Guide. 2005-09-20. Retrieved 2012-01-09.