A routing protocol that is used to exchange routing information between dynamic routers on Internet Protocol (IP) or Internetwork Packet Exchange (IPX) internetworks. Routing Information Protocol (RIP) was designed in 1980 to be used with the Xerox Network Systems (XNS) protocol suite but is most commonly used today in small to mid-sized TCP/IP internetworks.
RIP first became popular as a result of its inclusion in release 4.2 of the Berkeley BSD UNIX platform. It is supported by Microsoft Windows NT Server and Microsoft Windows 2000 Server and has been adapted to the AppleTalk networking system as the Routing Table Maintenance Protocol (RTMP)
RIP is based on the distance vector routing algorithm, one of several common routing algorithms that routers use to dynamically calculate the cost or metric of each possible path through an internetwork. RIP has been implemented for both TCP/IP and IPX/SPX internetworks as RIP for IP and RIP for IPX, respectively. RIP is designed for intradomain routing (routing within a flat routing space or routing domain).
Routing tables in RIP-enabled routers are calculated on the basis of the number of hops to the destination network. RIP routers do not use other routing metrics such as load, bandwidth, latency, or Maximum Transmission Unit (MTU) in calculating routing costs. The routing table of a RIP router contains the cost in hops of every path to every destination network in the internetwork.
RIP-enabled routers on a TCP/IP internetwork broadcast their complete routing tables every 30 seconds over User Datagram Protocol (UDP) port 520 using RIP advertisements. You might assume that this adds a lot of overhead to network traffic, but this information is broadcast information and is thus propagated only throughout the local network and received only by routers that have a routing interface to the local network. RIP does not support multipath routing. If a routing table has multiple routes for a single network ID, RIP stores the route with the lowest metric (number of hops to destination).
When a RIP router is first turned on, it announces its presence using a General RIP Request message so that neighboring RIP routers can send it advertisements of their routing tables. These RIP advertisements from neighboring RIP routers allow the router to build its own routing tables. In addition, the new RIP router announces to its neighbors all network IDs of locally attached networks so that they can update their routing tables with this information.
RIP supports a maximum metric of 15; networks that are more than 15 hops away are unreachable using RIP. This limits RIP implementation to small and mid-sized internetworks. RIP metrics are independent of TCP/IP Time to Live (TTL) values, so if two networks are separated by more than 15 routers on a RIP-enabled internetwork, packets sent between them are dropped even if their TTL values have not decremented to zero. If you try to send a packet to a network more than 15 hops away, a RIP router returns an Internet Control Message Protocol (ICMP) Destination Unreachable message.
RIP is a well-supported industry standard routing protocol. Its main disadvantage is that the routing table of a RIP-enabled router can be quite large because it contains all possible routes to all possible networks. RIP advertisement packets are only 512 bytes in length and can contain a maximum of 25 different routing table entries, so a large routing table with hundreds of entries means that dozens of RIP packets are broadcast every 30 seconds. This can result in a lot of extra broadcast traffic on the local subnet. RIP is therefore not suitable for large internetworks or for networks with many slow wide area network (WAN) links. RIP announcements are not synchronized over the internetwork and are sent without expectation of acknowledgments. This can lead to convergence problems and routing loops.
In addition, routing entries in a RIP routing table time out 3 minutes after the last RIP announcement is received, so if a RIP router goes down, it takes time for this information to propagate throughout the internetwork, a problem known as slow convergence. This 3-minute timeout value exists so that information about routers that unexpectedly fail or go down can be propagated throughout the internetwork. If neighboring routers do not hear from a RIP router within 3 minutes, networks that are locally attached to the missing router are assigned a hop count of 16, making them unreachable.
Microsoft Windows NT supports RIP for both TCP/IP (RIP for IP) and NWLink IPX/SPX-Compatible Transport (RIP for IPX). There are two versions of RIP:
A RIP-enabled router that can receive RIP broadcasts but cannot send them is called a Silent RIP Router.
Another disadvantage of RIP is that it cannot take into account real-time network parameters such as congestion, latency, or router load when the RIP router determines whether to forward a packet along a specific route. Use the Open Shortest Path First (OSPF) Protocol if you want to dynamically take into account such real-time network parameters, but be aware that implementing OSPF is fairly complex and might require you to upgrade your existing routers.
If your RIP-enabled internetwork includes slower WAN links as well as fast local area network (LAN) links between networks, you can assign the WAN links hop values that are greater than 1 to compensate for their slower speed. For example, you can assign a T1 link between two networks a hop count of 3 or 4. However, the total hop count between any two networks must still be less than or equal to 15, and such a configuration makes sense only if the topology of the network is a complex mesh involving both fast LAN and slow WAN links.
RIP routers should be turned off properly so that they can advertise the fact that they are being turned off to their neighboring routers. This notification, called a triggered update, declares all locally attached networks to the router as having a hop count of 16, making them unreachable. These triggered changes then propagate throughout the internetwork.