Understanding Selecting Network Protocols | OSPF Network Design Solutions

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RIP Timers

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:

• Routing update timer. Sets the interval between periodic routing updates—when the router sends a complete copy of its routing table to all neighbors. The default value for this timer is 30 seconds.

• Route invalid timer. If a router has not received an update about a particular route, the route might have become invalid. The route invalid timer determines the length of time that must expire before the indication “not heard” or the route is deemed invalid. After this occurs, the router’s neighbors are notified of the fact. The Cisco default value for this timer is 180 seconds.

• Route flush timer. This timer sets the interval between a route becoming invalid and its removal from the routing table. But before the router removes the invalid route’s entry from its table, the router must notify its neighbors. The default value for this timer is 240 seconds.

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.

Split Horizon

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:

1. Router 1 advertises that it has a route to Network A.

2. Router 2 receives the update from Router 1 and inserts the information about how to reach Network A in its routing table.

3. When Router 2 sends a regular routing update, it does not include the entry for Network A in the update sent to Router 1 because that route was learned from Router 1 in the first place. This procedure prevents a two-node routing loop, also known as split horizon.

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:

1. Router 1 advertises that it has a route to Network A.

2. Router 2 receives the update from Router 1 and inserts the information about how to reach Network A in its routing table.

3. Router 2 includes the entry for Network A in the update sent to Router 1, but the metric value is set to infinity, indicating that Network A is unreachable through Router 2 from Router 1.

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.

Hold-Down Mechanism

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:

1. A route goes down, and neighboring routers detect the failure.

2. These routers calculate new routes and send out routing update messages to inform neighbors of the route change.

3. A device that has yet to be informed of a network failure sends a regular update message indicating that the failed route is good.

4. This update reaches a device that has just been notified of the failure. That device now contains incorrect routing information, which it advertises in the routing updates it sends to its neighbors.

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.

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