Split Horizon and Holddown Timer

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Oct 10th, 2007
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With a distance vector protocol if the distance to a destination increases, the router sets a holddown timer until which the router will not accept any new updates for that route. Thats fair enough.

But I need to know if the holddown timer is still applicable when poison reverse is enabled also?

I mean when the router receives a rip update for a route (its own advertised route) with a metric of 16 due to poison reverse) will the router apply the holddown timer for the same route connected on another interface (with a metric lets say 2)?


Is the holddown used for updates with infinite metric (eg 16 for rip)?

Hope I made myself articulate enough.

Thanks in advance.

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onurcoskun Wed, 10/10/2007 - 01:03
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Also when providing redundancy with rip for example there are 2 links to the same route, the primary is an ethernet link with a metric of 1 and the secondary is a serial link with a metric of 3.

When the router first hears the secondary link update, does it enable the holddown timer during which the router normally wont accept any updated regarding the primary link?

GillieLucent Wed, 10/10/2007 - 03:08
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I am not able to understand your question completely. But, I have explained below splithorizon and Holddown timer.

There are two ways to avoid the count to infinity loop problem in RIP :

1.Split horizon with poisoned reverse

The essence of split horizon is the assumption that it is not useful to advertise routes learned from an interface back out to the network attached via that interface. With Poison reverse, it actually poisons the looped route by setting its metric to infinity.

2.Triggered updates

Split horizon with poisoned reverse will effectively prevent routing loops in topologies with just two gateways. In larger internetworks, however, RIP is still subject to the counting to infinity problem. To ensure that such infinite loops are caught as early as possible, RIP supports a triggered update.

A triggered update is a rule in the protocol that requires gateways to immediately broadcast an update message whenever it changes a route metric, regardless of how much time remains in the 30-second update timer.It is also possible (however unlikely) that another gateway could have just transmitted a periodic update before receiving a triggered update from another gateway. In this scenario, vestiges of an invalid route could repropagate throughout the network. A hold-down timer works in conjunction with the triggered update logic. In essence, as soon as a triggered update has been made, a clock starts counting down to zero. Until it decrements to zero, the router will not accept any updates from any neighbors for that route or destination.

This prevents a RIP router from accepting updates for a route that has been invalidated for a configurable amount of time. This prevents a router from being misled into believing that another router may have a viable route to an otherwise invalid destination.

Q : But I need to know if the holddown timer is still applicable when poison reverse is enabled also?

A : Holddown timer is not related to poison reverse, as it is counted only during triggered updates.

Q : Is the holddown used for updates with infinite metric (eg. 16 for rip)?

A : yes. Because, whenever it received an update with infinite metric, it sends triggered updated and starts holddown timer.




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