Chapter 6: Troubleshooting RIP Environments

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Routing Information Protocol (RIP) is one of the first distance vector routing protocols to use hop count as a metric to calculate the best routing path to a destination network. Hop count indicates the number of routers crossed to reach a destination network. RIP uses the Bellman-Ford algorithm to compute the metric used for routing path decision.

With time, networks became more complex, and the RIPv1 protocol was found to be inefficient in handling most of the functions. This incompetence was resolved with the release of RIPv2. Both RIPv1 and RIPv2 are simple in comparison to the new generation of routing protocols that includes EIGRP, OSPF, and IS-IS.

Features of RIP

Before getting into the problems associated with RIP, let us review some features specific to distance vector routing protocols and RIP.

The characteristics of RIP include:

• Classful routing protocol; that is, it will not send the subnet mask information about the network routes mentioned in the routing updates.

• Networks that implemented RIPv1 often encountered classful routing loops, because RIPv1 does not carry subnet information. RIPv2 addressed this shortcoming. It was designed to carry the subnet mask information by supporting VLSM.

• Deals with several timers—such as update timer, invalid timer, hold-down timer, and flush timer—that define the operation behavior of the RIP environment.

• Uses a UCP datagram to transmit routing updates. Each routing update can carry about 25 route entries.

• Enables routers to advertise their routing updates to their directly connected neighbors and can receive routing advertisements about other remote networks from its directly connected neighbors. The frequency of such updates is 30 seconds.

• Being a classful routing protocol, it is incapable of supporting Variable Length Subnet Masking (VLSM).

• Does not support discontiguous network; that is, the subnets of a major network cannot be separated by another major network.

• Runs into the issues of routing loops while in the process of building up the routing table due to its tendency to converge. This results in late convergence, because RIP-enabled routers learn routing information from their directly connected neighbor. To counter the problems related to routing loops, RIP uses solutions, such as maximum hop count, split horizon, route poisoning, poison reverse, hold-down timer, and triggered updates.

The characteristics of RIPv1 and RIPv2 are listed in Table 6.1.

Table 6.1 : Characteristics of RIPv1 and RIPv2

Feature	RIPv1	RIPv2
Category	Distance vector	Distance vector
Class type	Classful	Classless
VLSM	Not Supported	Supported
Authentication	Not Supported	Supported
Advertising address	Broadcast; 255.255.255.255	Multicast; 224.0.0.9
Autosummarization	Supported	Supported
Metric	Hop count	Hop count
Max hop count	15	15
Periodic interval	30 seconds	30 seconds

Though RIP is the one of the simplest routing protocols to deploy, there are numerous issues that lead to its unpredictable behavior. By understanding the basic functionalities and following a few guidelines, you will be successful in eliminating most of the RIP problems. The common problems encountered in RIP environments are:

• Misconfiguration

• Classless routing

• Timers

• Looping

• Version problems; incompatibility issues