Among these issues are routing loops and counting to infinity. These result in inconsistencies due to routing update messages with out of date routes being propagated around the internetwork.

To reduce routing loops and counting to infinity, RIP uses the following techniques:

• Count-to-infinity
• Split horizon
• Poison reverse
• Holddown counters
• Triggered updates

Some of these methods may require some configuration, while others do not require or rarely require configuration.

RIP permits a maximum hop count of 15. Any destination greater that 15 hops away is tagged as unreachable. RIP’s maximum hop count greatly restricts its use in large internetworks but prevents a problem called “counting to infinity” from causing endless network routing loops.

The split horizon rule is based on the theory that it is not useful to send information about a route back in the direction from which it came. In some network configurations, it may be necessary to disable split horizon.

The following command is used to disable split horizon:

GAD(config-if)#no ip split-horizon

The holddown timer is another mechanism that may need some changes. Holddown timers help prevent counting to infinity but also increase convergence time. The default holddown for RIP is 180 seconds. This will prevent any inferior route from being updated but may also prevent a valid alternative route from being installed. The holddown timer can be decreased to speed up convergence but should be done with caution. The ideal setting would be to set the timer just longer that the longest possible update time for the internetwork. In the example in Figure , the loop consists of four routers. If each router has an update time of 30 seconds, the longest loop would be 120 seconds. Therefore, the holddown timer should be set to slightly more than 120 seconds.

To change the holddown timer:

Router(config-router)#timers basic update invalid holddown flush [sleeptime]

One additional item that affects convergence time, and is configurable, is the update interval. The default RIP update interval in Cisco IOS is 30 seconds. This can be configured for longer intervals to conserve bandwidth, or for shorter intervals to decrease convergence time.

To change the update internal:

GAD(config-router)#update-timer seconds

Another issue with routing protocols is the unwanted advertisement of routing updates out a particular interface. When a network command is issued for a given network, RIP will immediately begin sending advertisements out all interfaces within the specified network address range. To control the set of interfaces that will exchange routing updates, the network administrator can disable the sending of routing updates on specified interfaces by configuring the passive-interface command.

Because RIP is a broadcast protocol, the network administrator may have to configure RIP to exchange routing information in a non-broadcast network such as Frame Relay. In this type of network, RIP needs to be told of other neighboring RIP routers. To do this use the command displayed in Figure .

By default, the Cisco IOS software receives RIP Version 1 and Version 2 packets, but sends only Version 1 packets. The network administrator can configure the router to only receive and send Version 1 packets or the administrator can configure the router to send only Version 2 packets. To configure the router to send and receive packets from only one version, use the commands in Figure .

To control how packets received from an interface are processed, use the commands in Figure .

Web Links Configuring RIP http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/ products_configuration_guide_ chapter09186a00800d97f7.html#xtocid1