Introduction

RIP Version 1 was the Internet's first widely used routing protocol. It was standardized in RFC 1058, although implementations of the protocol based on de facto standards existed much earlier. It is still useful in small, simple networks. RIP Version 2 is documented in RFC 1723. All Cisco routers support RIP Version 1. Version 2 support was integrated into IOS Version 11.1. A detailed discussion of RIP Versions 1 and 2 is beyond the scope of this book. If you are unfamiliar with dynamic routing protocols in general or with RIP in particular, you can find theoretical descriptions of how the protocol works in IP Routing (O'Reilly) and Designing Large-Scale LANs (O'Reilly). We also recommend reading the appropriate RFCs.

RIP is useful in some situations, but you have to remember its limitations. First, it is a purely classful protocol, and Version 1 doesn't support variable length subnet masks. So you should not use this protocol if you do any complex subnetting. Second, both Versions 1 and 2 of RIP use the very small metric value of 16 to signify "infinity." The protocol considers any network that is more than 16 hops away to be unreachable. This is particularly important if you adjust any metric values to make RIP favor a fast link over a slow one. In practice, it is quite easy to exceed the maximum metric, even in small networks.

However, RIP can be extremely useful over small parts of a network. For example, it is much easier to configure than BGP as a method for interconnecting two or more different OSPF or EIGRP Autonomous Systems. And, because RIP has been around for so long, it is often useful when exchanging routing information with legacy equipment. Indeed, it is almost impossible to find a router of any age from any vendor that doesn't implement RIP.

In this book, we assume that you are familiar with RIP in general and focus on Cisco's implementation of it. We also discuss some specific issues that we think are particularly important.

One of the central features of RIP is that it distributes the entire routing table every 30 seconds. The protocol requires every device to add a small random amount to this time period, but doesn't specify the size of this offset, or whether it should be positive or negative. Cisco routers always reduce this period slightly by subtracting a random variable amount of time, up to 4.5 seconds. This helps to prevent the synchronization problems caused by several routers sending their updates simultaneously, which can in turn cause network loading problems.

If a particular route is not seen for 6 successive update cycles, or 180 seconds by default, the routers will mark it as invalid. They will then flush the invalid route from their routing tables if they don't see it for 8 cycles, or 240 seconds. This implies that RIP is slow to converge after a failure, but the network will actually converge much more quickly if it can take advantage of a protocol feature called triggered updates. This means that when a route's metric suddenly changes, for whatever reason, a router using triggered updates will not wait for the full update cycle before distributing information about the change to the other routers in the network.

This is different from the modification to RIP described in Recipe 6.11, which is also called a triggered update. This feature, which is a partial implementation of RFC 2091, allows the routers to send routing updates only when there are changes, instead of sending the entire routing table at each update cycle. This makes it possible to configure the routers to just send incremental changes. Using triggered updates drastically improves RIP performance, but it must be configured on all of the routers sharing the link. It is often unsupported by legacy equipment, and Cisco routers support this feature only on point-to-point serial links.

Cisco routers implement a hold-down timer with RIP. This is a protocol feature that is not described in the standard protocol RFCs. When the router marks a route invalid, it starts the hold-down timer, which is 180 seconds by default, and ignores all updates for this route. This helps to make the network somewhat more stable.

Because RIP uses a distance vector algorithm rather than a link state protocol (like OSPF), you can use Cisco's distribute lists to make a router distribute only certain routes. This allows you to prevent distribution of routing information that you don't want to be generally visible. And you can also reject incoming routing information that you don't want to use. This can be extremely useful when exchanging routing information between networks, or when connecting small networks' regions with legacy equipment.