Routing Information Protocol - A Brief Theory

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The Routing Information Protocol (RIP) is a comparatively old interior gateway protocol (IGP) still in widespread use. IGPs are used for routing within networks that are under a common network administration, whereas exterior gateway protocols are used to exchange routing information between networks. As an IGP, RIP performs routing only within a single autonomous system. RIP is a classical distance vector routing protocol that uses hop count as its metric for determining the best route to a given destination.

RIP Routing Updates

RIP sends routing update messages at regular 30-second intervals and when the network topology changes. RIP uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. The routing update process is termedadvertising. When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route. The metric value for the path is increased by 1, and the sender is indicated as the next hop. RIP routers maintain only the best route (the route with the lowest metric value) to a destination. After updating its routing table, the router immediately begins transmitting routing updates to inform other network routers of the change. These updates are sent in addition to the regularly scheduled 30-second interval updates that RIP routers send.

RIP Routing Metric

RIP uses a single routing metric (hop count) to measure the distance between the source and a destination network. Each hop in a path from source to destination is assigned a hop-count value, which is 1. When a router receives a routing update that contains a new or changed destination-network entry, the router adds 1 to the metric value indicated in the update and enters the network in the routing table. The IP address of the sender is used as the next hop.

RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops in a path is 15. If a router receives a routing update that contains a new or changed entry, and if increasing the metric value by 1 causes the metric to be infinity (defined as 16), the network destination is considered unreachable.

RIP Scalability and Limitations

The low hop count of RIP is considered a scalability limitation for large networks. Another limitation is that RIP Version 1 (RIP-1) is a classful routing protocol and does not carry subnet mask information in its routing updates.

NOTE
RIP Version 2 (RIP-2) was introduced to address this limitation. The RIP-2 specification (described in RFC 1723) allows more information, such as the subnet mask, to be included in RIP packets and provides a simple authentication mechanism.

Because of this, RIP-1 does not support the use of variable-length subnet masking (VLSM). VLSM provides the capability to specify a different subnet mask for the same network number, but on different subnets. Before RIP-1 sends out an update, it performs a check against the subnet mask of the network that is about to be advertised. If a VLSM has been assigned, the subnet gets dropped from the advertisement. This limitation also poses scalability issues for large networks in which address space is limited.

RIP Stability Features

RIP implements mechanisms such as split horizon, hold-down timers, hop-count limits, and poison reverse to prevent routing loops and maintain network stability, as explained in the list that follows:
  • Split horizon—If a route is learned on an interface, the information about that route is not sent back out the interface where it was learned. In this way, split horizon prevents routing loops within the network.
  • Hold-down timers—These timers ignore routing update information for a specified period of time. Hold-down timers can be reset when the timer expires, a routing update is received that has a better metric, or a routing update is received indicating that the original route to the network is valid. Hold-down timers are useful in preventing routing information from flooding the network when network links are unstable.
  • Hop-count limit—This limits the number of hops allowed in a path from source to destination. The maximum is 15, and 16 is deemed unreachable. The hop-count limit prevents routing loops from continuing indefinitely.
  • Poison reverse—A route is "poisoned" when a router marks a route as unreachable by setting the hop count to 16 and then passes this route out to a neighboring router, causing the neighboring router to remove the route from its routing table. This speeds network convergence by preventing invalid routes from being propagated throughout the network.
These features allow RIP to adjust to network-topology changes and prevent routing loops from being propagated and continuing indefinitely.

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