Chapter 2. Understanding Routing Information Protocol (RIP)

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Chapter 2. Understanding Routing Information Protocol (RIP)

This chapter covers the following key topics about Routing Information Protocol (RIP):

  • Metric

  • Timers

  • Split horizon

  • Split horizon with poison reverse

  • RIP-1 packet format

  • RIP behavior

  • Why RIP doesn't support discontiguous networks

  • Why RIP doesn't support variable-length subnet masking (VLSM)

  • Default routes and RIP

  • Protocol extension to RIP

  • Compatibility issues

RIP is a distance vector protocol that uses hop count as its metric. This protocol is very simple and was intended for small networks. RIP is similar to gated, which was distributed by the FreeBSD version of UNIX. Before the RFC for RIP Version 1 (RIP-1) was written, several versions of RIP were floating around.

NOTE

Hop count refers to the number of routers being traversed. For example, a hop count of 2 means that the destination is two routers away.


RIP is a classful protocol, which means that it doesn't carry subnet mask information in its routing update. Because it doesn't carry any subnet mask information, it is incapable of supporting variable-length subnet masking (VLSM) and discontiguous networks. RIP enables devices to exchange information about networks that they are directly connected to, as well as any other networks that they have learned from other RIP devices.

RIP sends its routing information every 30 seconds, which is the default update timer. This timer is configurable. The hold-down timer determines how long a router should wait before flushing the information from the routing table.

RFC 1058 was written to provide a standard for RIP, which uses the Bellman-Ford algo-rithm to compute its metric.

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Metric

The RIP metric is based on hop count and can be between 1 and 15. The metric 16 is used for infinity, which means that if the route is unreachable, a metric of 16 is displayed. The question is, why was the metric chosen as 16? Why not 17 or 18? The metric filed in RIP-1 packet format clearly shows that it is 32 bits long. This means that, theoretically, RIP can support 2 32 hops. Although this is a large number, the metric of 15 was chosen to avoid a count to infinity problem. (This is also referred to as a routing loop. ) In a large network with a few hundred routers, a routing loop results in a long time for convergence if the metric for infinity has a large value. The number 16 was chosen to get a shorter convergence time.

The 15-hop limit was chosen also because RIP was intentionally designed for small networks. It was not intended for the large networks that potentially can have more than 15 hops.

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Timers

Like any distance vector protocol, RIP periodically sends an update every 30 seconds. This update consists of a broadcast of the entire routing table. The update timer controls this 30-second period. RIP uses the following timers:

  • Update ‚ The time between each update interval. This value is set to 30 seconds, by default, and is configurable.

  • Invalid ‚ The time after which a suspect route becomes invalid. This is set to 180 seconds, by default.

  • Hold-down ‚ The time used to suppress the possibility of defective routes being installed in the routing table. The default time is 180 seconds.

  • Flush ‚ The time after which a route is removed from the routing table. This is set to 240 seconds, by default.

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