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EIGRP uses DUAL to achieve route-loop freedom. By design DUAL ensure no route-loop in the network.

  1. Then why does EIGRP use poison reverse UPDATE when there is a topology change? Is there any other reason to send poison reverse other than route-loop avoidance?
  2. Furthermore, why do we use poison reverse instead of split horizon technique? ie not to send an UPDATE (poisoned) back on the interface on which it learnt about a destination.
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    Poison revers is used with split horizon. Cisco documents explain it:cisco.com/c/en/us/support/docs/ip/…
    – Ron Maupin
    Feb 9 at 17:16
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    item 2 makes no sense, poison reverse means to send Update (d = inf)
    – Effie
    Feb 9 at 18:40
  • Isn't split horizon and poison reverse two different mechanism to avoid routing loops? Split horizon says that an update should not be sent on the intf on which it learnt about a destination. In poison reverse, we send an update on the same intf on which we learnt about the destination. Only that we send the update with infinite metric.
    – Darshan L
    Feb 10 at 10:06
  • Yes, split horizon says that you do not send an update to the interface that sent you the update, but poison reverse is not send the same update out the interface on which it was received; it is sending your own update to the source of the update, telling the source that you no longer have a route to the destination. Both split horizon and poison reverse are used in conjunction. Neither really works without the other.
    – Ron Maupin
    Feb 15 at 1:19
  • @RonMaupin - Okay. Why does a router send a poison reverse to the router from which it learnt about a destination? ie why should a router (B) inform the source of the update router (A) that it no longer have a route to the destination? Wouldn't it be enough if the router (B) stayed quite without sending any UPDATE (poisoned route) to the source of the update router (A)? which essentially makes the source of the update router (A) to not consider router (B) as a path to the destination.
    – Darshan L
    Feb 15 at 18:29

1 Answer 1

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Poison reverse is used with split horizon. It helps prevent loops

There is a Cisco document that explains it all

(Hat tip to @ronmaupin)

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  • Isn't split horizon and poison reverse two different mechanism to avoid routing loops? Split horizon says that an update should not be sent on the intf on which it learnt about a destination. In poison reverse, we send an update on the same intf on which we learnt about the destination. Only that we send the update with infinite metric.
    – Darshan L
    Feb 10 at 10:06

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