What does the RIP holddown timer do?

ensures an invalid route has a metric of 15
prevents a router from sending any updates after it has introduced a routing loop into the network
ensures every new route is valid before sending an update
instructs routers to ignore updates, for a specified time or event, about possible inaccessible routes

ccna exploration 2 chapter 4 Exam Question And Answers 

What does the RIP holddown timer do?