Routing Information Protocol (RIP)

One of the most widely used interior gateway protocols is the Routing Information Protocol (RIP). RIP is an implementation of a distance-vector, or Bellman-Ford, routing protocol for local networks. It classifies routers as active and passive (silent). Active routers advertise their routes (reachability information) to others; passive routers listen and update their routes based on advertisements but do not advertise. Typically, routers run RIP in active mode, while hosts use passive mode.

A router running RIP in active mode broadcasts updates at set intervals. Each update contains paired values where each pair consists of an IP network address and an integer distance to that network. RIP uses a hop count metric to measure the distance to a destination. In the RIP metric, a router advertises
directly connected networks at a metric of 1. Networks that are reachable through one other gateway are two hops. Thus, the number of hops or the hop count along a path from a given source to a given destination refers to the number of gateways that a datagram would encounter along that path. Using hop counts to calculate shortest paths does not always produce optimal results. For example, a path with hop count of 3 that crosses three Ethernets may be substantially faster than a path with a hop count of 2 that crosses two slow-speed serial lines. To compensate for differences in technology, many routers advertise artificially high hop counts for slow links.

The routing daemons that run RIP dynamically build on information received through RIP updates.
When started up, the daemon issues a REQUEST for routing information and then listens for responses to the request. If a system configured to supply RIP hears the request, it responds with a RESPONSE packet based on information in its routing database. The RESPONSE packet contains destination network addresses and the routing metric for each destination.

When a RIP RESPONSE packet is received, the routing daemon takes the information and rebuilds the routing database, adding new routes and "better" (lower metric) routes to destinations already listed in the database. RIP also deletes routes from the database if the next router to that destination says the route contains more than 15 hops, or if the route is deleted.

All routes through a gateway are deleted if no updates are received from that gateway for a specified time period. In general, routing updates are issued every 30 seconds. In many implementations, if a gateway is not heard from for 180 seconds, all routes from that gateway are deleted from the routing database.

RIP Version 2 (more commonly known as RIP II) adds additional capabilities to RIP. Some of these capabilities are compatible with RIP I and some are not.
To avoid supplying information to RIP I routes that could be misinterpreted, RIP II can only use incompatible features when its packets are multicast. On interfaces that are not capable of IP multicast, RIP-I-compatible packets are used that do not contain potentially confusing information.

Some of the most notable RIP II enhancements are:

next hop

This is the ability to advertise a next hop to use other than the router supplying the routing update. This is quite useful when advertising a static route to a dumb router that does not run RIP as it avoids having packets destined through the dumb router from having to cross a network twice.

Although next hop information is provided in RIP I-compatible RIP II packets, RIP I routers ignore next hop information in RIP II packets. This may result in packets crossing a network twice, which is the default behavior with RIP I.

network mask

RIP I assumes that all subnetworks of a given network have the same network mask. It uses this assumption to calculate the network masks for all routes received. This assumption prevents subnets with different netmasks from being included in RIP packets. RIP II adds the ability to explicitly specify the network mask for each network in a packet.

While RIP I routers will ignore the network mask in RIP II packets, their calculation of the network mask will quite possibly be wrong. For this reason, RIP I-compatible RIP II packets must not contain networks that would be misinterpreted. These networks must only be provided in native RIP II packets that are multicast.

authentication

RIP II packets may also contain an authentication string that can be used to verify the validity of the supplied routing data. The simple password method used provides very little security in the presence of network monitoring software. Authentication can be used in RIP I-compatible RIP II packets,

but be aware that RIP I routers will ignore it.

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