Common RIP configuration issues

Common RIP configuration issues 

7.2.4 

This page will provide some more information about the methods that are used to reduce routing loops.
RIP routers must rely on neighbor routers for some types of network information. A common term used to describe this functionality is Routing by Rumor. RIP uses a distance vector routing algorithm. All distance vector routing protocols have issues that are primarily created by slow convergence. Convergence is when all routers in a network have the same routing information.
Among these issues are routing loops and counting to infinity. These result in inconsistencies due to update messages with incorrect routes that are propagated around the network.
To reduce routing loops and counting to infinity, RIP uses the following techniques:

• Split horizon
• Poison reverse
• Holddown counters
• Triggered updates

Some of these methods may need to be configured.
RIP permits a maximum hop count of 15. Any destination greater that 15 hops away is tagged as unreachable. This maximum hop count greatly restricts the use of RIP in large internetworks but prevents counts to infinity and 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 to be configured. 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. Ideally, the timer should be set just longer than 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.
Use the following command to change the holddown timer as well as the update, invalid, and flush timers:

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

Another configurable item that affects convergence time 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.
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 to send advertisements out all interfaces within the specified network address range. A network administrator can use the passive-interface command to disable routing updates on specified interfaces.  
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 must be informed of neighbor RIP routers. To do this use the neighbor 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 .
The next page will show students how to verify a RIP configuration.

Using the ip classless command

Using the ip classless command 

7.2.3

This page will explain what the ip classless command is and how it is used.
Sometimes a router receives packets destined for an unknown subnet of a network that has directly connected subnets. Use the ip classless global configuration command to instruct the Cisco IOS software to forward these packets to the best supernet route. A supernet route is a route that covers a greater range of subnets with a single entry. For example, if an enterprise uses the entire subnet 10.10.0.0 /16, then a supernet route for 10.10.10.0 /24 would be 10.10.0.0 /16. The ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later. To disable this feature, use the no form of this command.
When this feature is disabled any packets received that are destined for a subnet that falls within the subnetwork addressing scheme of the router will be discarded.
IP classless only affects the operation of the forwarding processes in IOS. IP classless does not affect the way the routing table is built. This is the essence of classful routing. If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped.
The most confusing aspect of this rule is that the router only uses the default route if the major network destination does not exist in the routing table. A router by default assumes that all subnets of a directly connected network should be present in the routing table. If a packet is received with an unknown destination address within an unknown subnet of a directly attached network, the router assumes that the subnet does not exist. So the router will drop the packet even if there is a default route. To resolve this problem, configure ip classless on the router. This allows the router to ignore the classful boundaries of the networks in its routing table and simply route to the default route. -
The Lab Activity will help students become more familiar with the ip classless command.
The next page describes some methods that are used to reduce routing loops.

RIP Configuring

RIP Configuring

7.2.2  This page will explain how RIP is configured.
The router rip command enables RIP as the routing protocol. The network command is then used to tell the router on which interfaces to run RIP. The routing process associates specific interfaces with the network addresses and begins to send and receive RIP updates on these interfaces.
RIP sends routing-update messages at regular intervals. When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route. The received metric value for the path is increased by 1, and the source interface of the update is indicated as the next hop in the routing table. RIP routers maintain only the best route to a destination but can maintain multiple equal-cost paths to the destination. Most routing protocols use a combination of time-driven and event-driven updates. RIP is time-driven, but the Cisco implementation of RIP sends triggered updates whenever a change is detected. Topology changes also trigger immediate updates in IGRP routers, regardless of the update timer. Without triggered updates, RIP and IGRP will not perform. After updating its routing table due to a configuration change, the router immediately begins transmitting routing updates in order to inform other network routers of the change. These updates, called triggered updates, are sent independently of the regularly scheduled updates that RIP routers forward. The descriptions for the commands used to configure router BHM shown in the figure are as follows:

• BHM(config)#router rip – Selects RIP as the routing protocol
• BHM(config-router)#network 10.0.0.0 – Specifies a directly connected network
• BHM(config-router)#network 192.168.13.0 – Specifies a directly connected network

The Cisco router interfaces that are connected to networks 10.0.0.0 and 192.168.13.0 send and receive RIP updates. These routing updates allow the router to learn the network topology from a directly connected router that also runs RIP.
RIP must be enabled and the networks must be specified. All other tasks are optional. These optional tasks include the following:

• Apply offsets to routing metrics
• Adjust timers
• Specify a RIP version
• Enable RIP authentication
• Configure route summarization on an interface
• Verify IP route summarization
• Disable automatic route summarization
• Run IGRP and RIP concurrently
• Disable the validation of source IP addresses
• Enable or disable split horizon
• Connect RIP to a WAN

To enable RIP, use the following commands in global configuration mode:

• Router(config)#router rip – Enables the RIP routing process
• Router(config-router)#networknetwork-number – Associates a network with the RIP routing process

The Lab Activities will let students examine a router topology and configure RIP.
The next page will introduce the ip classless command

RIP / RIP routing process

RIP 

RIP routing process 

7.2.1
This page will provide an overview of the RIP routing process.
The modern open standard version of RIP, which is sometimes referred to as IP RIP, is formally detailed in two separate documents. The first is known as Request for Comments (RFC) 1058 and the other as Internet Standard (STD) 56.
RIP has evolved over the years from a Classful Routing Protocol, RIP Version 1 (RIP v1), to a Classless Routing Protocol, RIP Version 2 (RIP v2). RIP v2 enhancements include the following:

• Ability to carry additional packet routing information
• Authentication mechanism to secure table updates
• Support for variable-length subnet mask (VLSM)

To prevent indefinite routing loops, RIP implements a limit on the number of hops allowed in a path from a source to a destination. The maximum number of hops in a path is 15. When a router receives a routing update that contains a new or changed entry, the metric value is increased by 1 to account for itself as a hop in the path. If this causes the metric to be higher than 15, the network destination is considered unreachable. RIP includes a number of features that are common in other routing protocols. For example, RIP implements split horizon and holddown mechanisms to prevent the propagation of incorrect routing information.
The next page will teach students how to configure RIP.