RIP has a number of configurable timers that enable the network engineer an added amount of flexibility when implementing RIP. These timers are also very good tools with which to monitor and regulate RIP performance. These timers are as follows:
Increasing RIP Stability
RIP specifies a number of stability features to adjust for rapid network topology changes. These features are common to many routing protocols.
For example, RIP implements the split horizon and hold-down mechanisms to prevent incorrect routing information from being propagated. In addition, the RIP hop count limit prevents routing loops from continuing indefinitely.
The split horizon rule states that it is never useful for a routing protocol to send information about a route back to the router from which it was learned. Split horizon helps prevent two-node routing loops. The split horizon rule takes two forms: simple split horizon and split horizon with poison reverse.
Simple Split Horizon
The simple form of the split horizon rule simply states that routing updates sent to a particular neighbor router should not contain information about routes that were learned from that neighbor.
An example of how RIP operates with simple split horizon implemented is as follows:
Split Horizon with Poison Reverse
The poison reverse form of the split horizon rule allows routing updates sent to a particular neighbor router to include information about routes learned from that neighbor. However, the metric for these routes is set to infinity.
An example of how RIP operates with split horizon with poisoned reverse implemented is as follows:
Split horizon with poisoned reverse is usually preferred to simple split horizon. A routing loop is broken immediately if a router receives an update that sets the metric of a route to infinity. With simple split horizon, the routing loop will not be broken until a boundary is imposed, such as a hop count limit or a timer expiration.
The disadvantage of poisoned reverse is that it can greatly increase the size of routing messages, often simply to advertise multiple unreachable networks.
The hold-down mechanism prevents regular routing updates from inappropriately reinstating incorrect routing information. When a router receives an update that contains a topology change (that is, invalid), it starts the hold-down timer.
The hold-down timer prevents a router from implementing any change to its routing table until the timer expires. Any update received during this period is discarded. The hold-down period is usually slightly longer than the time necessary for the entire network to converge on a topology change.
In the following scenario, incorrect routing information is advertised because the hold-down mechanism is not implemented:
The hold-down mechanism solves this problem by forcing every router to retain routing information changes for as long as it takes for all routers in the internetwork to converge on the change.
Hop Count Limit
RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops in a path is 15.
If a router receives a routing update that contains a new or changed entry, and increasing the metric value by one causes the metric to be infinity (that is, 16), the network destination is considered unreachable.