RIP

Introduction

The Routing Information Protocol ( RIP) is an interior gateway protocol (IGP) that was developed as part of the ARPANET project and was included in the Unix BSD operating system in the early 1980s. RIP was widely deployed in the 1980s and became the industry standard for interior routing. It was standardized by the IETF in 1988, in RFC 1058. This version is referred to as RIP Version 1. RIP Version 2, defined in RFC 2453, added support for Classless Interdomain Routing ( CIDR) and authentication. RIP Version 2 MD5 authentication is defined in RFC 2082. RFCs 2080 and 2081 define RIPng, which is designed for IPv6 networks. JUNOS software supports RIP Versions 1 and 2, and RIPng.

RIP is a relatively simple protocol. It uses a distance-vector algorithm (also called the Bellman-Ford algorithm) to determine the best route to a destination. The distance is measured in hops, which is the number of routers that a packet must pass through to reach the destination. The best route is the one with the shortest number of hops. In the routing table, the router maintains two basic pieces of information for RIP routes: the IP address of the destination network or host and the hop count (metric) to that destination.

Every 30 seconds, devices on a RIP network broadcast RIP route information, which describes their view of the network topology and generates a lot of traffic on the network. RIP uses two techniques to reduce the amount of traffic:

 

Split horizon

A device receives a route advertisement on an interface but does not retransmit that advertisement back on the same interface. This limits the amount of RIP traffic by eliminating information that its RIP neighbor has already learned.

 

Poison reverse

If a RIP device learns from an interface that a device is no longer connected or reachable, it advertises that device's route back on the same interface, setting the number of hops to 16, which means infinite or unreachable. Poison reverse improves the convergence time on a RIP network.

By default, the JUNOS software implements both split horizon and poison reverse.

For service provider networks that use Juniper Networks routers, IS-IS or OSPF are generally used for the IGP because they are more powerful routing protocols and have more features for the larger service provider networks. You might have to use RIP for part of your network if it still has devices running RIP or for one of your customers if they still have devices running RIP. You might choose to use RIP because it is a relatively simple protocol, has very few advanced features, and is relatively straightforward to configure and manage. RIP can be useful in a small, reasonably homogeneous network, which might be served by some of the newer, smaller J-series routers.

This chapter discusses how to enable RIP on the router, how to set it up to receive and send both Version 1 and Version 2 protocol update packets, how to set up some simple routing policies to filter the traffic that RIP sends and receives, and how to perform basic troubleshooting of RIP traffic.

If you use RIP, you should remember that the protocol itself has some inherent limitations. RIP can be used only in small networks because the maximum number of hops to a destination is 16. If a RIP device is more than 15 hops away, it is considered to be unreachable. In practice, this is often a serious limitation. From a route convergence point of view, you should use RIP only if your network is small, with no devices more than four hops from each other. If the network diameter is larger than this, the route convergence time increases to about two to four minutes, which can lead to network instabilities and routers becoming unreachable. In comparison, OSPF and IS-IS typically converge in about 40 seconds. Although it is possible to influence the convergence times by altering RIP timers, if you find yourself having to do this, you should consider using OSPF or IS-IS instead of RIP.

RIP Version 1 has two additional limitations. First, it uses only classful routing, so it cannot handle subnet and network mask information. Second, it uses clear-text password authentication, which is vulnerable to attack. RIP Version 2 was developed to address these two limitations, supporting CIDR and MD5 authentication. However, the hop-count limit of 15 was retained to maintain interoperability with Version 1.

The JUNOS implementation of RIP also has a design point of note. By default, the JUNOS RIP only listens to RIP updates. The router does not send RIP updates unless you explicitly tell RIP to do so. You do this by creating a routing policy, which can be a fairly simple policy, to advertise the routes.