RIP (Routing Information Protocol)

1) What is RIP?
Answer: RIP is the dynamic routing information protocol used in local as well as wide network.
RIP is a distance-vector routing protocol, which employs the hop count as a routing metric. The hold down time is 180 seconds. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15. This hop limit, however, also limits the size of networks that RIP can support. A hop count of 16 is considered an infinite distance and used to deprecate inaccessible, inoperable, or otherwise undesirable routes in the selection process.

2) What are the types of RIP and there difference?
Answer: RIP has three version
RIPv1: It used broadcast address and has no support for router authentication.

RIPv2: It used multicast arress and support MD5 authentication .

RIPng:

RIPng (RIP next generation), is an extension of RIPv2 for support of IPv6, the next generation Internet Protocol. The main differences between RIPv2 and RIPng are:

• Support of IPv6 networking.
• While RIPv2 supports RIPv1 updates authentication, RIPng does not. IPv6 routers were, at the time, supposed to use IPsec for authentication.
• RIPv2 allows attaching arbitrary tags to routes, RIPng does not;
• RIPv2 encodes the next-hop into each route entries, RIPng requires specific encoding of the next hop for a set of route entries.

3) What is Route poisoning ?
Answer:

Route poisoning is a method to prevent routing loop within computer network. Distance vector routing protocol in computer networks use route poisoning to indicate to other routers that a route is no longer reachable and should be removed from their routing tables. A variation of route poisoning is split horizon with poison reverse whereby a router sends updates with unreachable hop counts back to the sender for every route received to help prevent routing loops.

When the protocol detects an invalid route, all of the routers in the network are informed that the bad route has an infinite (∞) route metric. This makes all nodes on the invalid route seem infinitely distant, resulting in preventing any of the routers from sending packet over the invalid route.

Some distance-vector routing protocols, such as RIP, use a maximum hop count to determine how many routers traffic must go through to reach the destination. Each route has a hop count number assigned to it which is incremented as the routing information is passed from router to router. A route is considered unreachable if the hop count exceeds the maximum allowed. Route poisoning is a method of quickly removing outdated routing information from other router's routing tables by changing its hop count to be unreachable (higher than the maximum number of hops allowed) and sending a routing update.

In the case of RIP, the maximum hop count is 15, so to perform route poisoning on a route its hop count is changed to 16, deeming it unreachable, and a routing update is sent.

In some routing protocols, each router advertises routes it has learned from a neighboring router back to the router the advertisement came from with an infinite route metric; this is called split horizon with poison reverse. This is done to reduce the possibility of loops and reduce route convergence time.

4) What are the mechanism to prevent the incorrect routing information in RIP?

Answer: RIP implements the split horizon, route poisoning and holddown mechanisms to prevent incorrect routing information from being propagated.

Holddown timer works by having each router start a timer when they first receive information about a network that is unreachable. Until the timer expires, the router will discard any subsequent route messages that indicate the route is in fact reachable. It can solve the case where multiple routers are connected indirectly. The classic example would be three routers configured in a triangle. In this situation,Split horizon and split horizon with poisoned reverse can do nothing.

In other words, a holddown keeps a router from receiving route until the network appears to be stable—until either an interface stops changing state (flapping) or a better route is learned.

Holddowns are usually implemented with timers. If the router detects a network unreachable, the timer is started. The router will then wait a present number of seconds until the network is stable. When the timer expires, the router will then receive its routing updates from other routers. For example in RIP the default holddown timer is set on 180 seconds.