Split Horizon And Poison Reverse

up2thetime

Hey guys... I was noticing the way split horizon and poison reverse works, and to me, it seems somewhat redundant...

Ill explain with an example:

R1 is advertising 172.16.22.0/24 to R2 via RIP. Split horizon prevents R2 from advertising this network back to R1. All of a sudden the network goes down and R1 sends a triggered update to R2 with a metric of 16 (R1 will poison the route).

Here's where it seems redundant... Upon receiving a metric 16 advertisement from R1, R2 suspends split horizon rules and advertises 172.16.22.0/24 back to R1 with a metric of 16 (R2 does a poison reverse).

What is the point of the poison reverse step since split horizon prevents R2 from advertising this subnet back to R1 in the first place.....?

Thanx!

Monkerz

The poison reverse is just to indicate to R1 that R2 has received the poison. An ACK, if you will.

When a router receives a route poisoning, it sends an update back to the router from which it received the route poisoning, this is called poison reverse. This is to ensure that all routers on a segment have received the poisoned route information.

up2thetime

Hey thanks!
Did some labbing in GNS/Wireshark. Looks like R2 doesn't just send the poison reverse back to R1. If R2 is connected to other segments, it will advertise 172.16.22.0/24 on those other segments with an infinite metric.