Debug IP RIP - suppressing null update

Several of my CIT1122 students wanted to know, when they issued the command, "debug ip rip", what it meant when they saw, "suppressing null update" in the output when RIP was building the route entries to broadcast out to a neighbor.

This is normal behavior with rip when there is no need to include a particular route - either the route is connected and shared between the two routers and there is no need to advertise; the route(s) are learned already from that interface with a better metric than what you have to send; or it could be due to the split horizon rule - specific routes in the routing table are being suppressed because they were learned by that same interface (therefore split horizon says don't send them back out the that same interface).

Hope that helps - Joanne