Understanding Selecting Network Protocols | OSPF Network Design Solutions

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Routing Information Protocol (RIP)

The Routing Information Protocol (RIP) is a distance vector protocol that practices classful routing. The original incarnation of RIP was the Xerox protocol, GWINFO. A later version shipped with Berkeley Standard Distribution (BSD) UNIX in 1982.

RIP evolved as an Internet routing protocol. Other protocol suites use modified versions of RIP. For example, the AppleTalk Routing Table Maintenance Protocol (RTMP) and the Banyan VINES Routing Table Protocol (RTP) are both based on the Internet Protocol (IP) version of RIP.

The latest enhancement to RIP is the RIPv2 specification, which enables more information to be included in RIP packets and provides a simple authentication mechanism. The Routing Information Protocol (RIP) is a relatively old, but still commonly used, Interior Gateway Protocol (IGP) created to perform routing within a single autonomous system.

RIP Specifications

RIP is a classical distance vector routing protocol. RIP is primarily documented in RFC 1058. RIP uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. RIP is specified in the following Requests for Comments (RFCs): RFC 1058 and RFC 1723.

Key Characteristics of RIP

Some of the notable characteristics of RIP include the following:

• Distance vector routing protocol

• Hop count is the only metric used for path selection

• Maximum allowable hop count is 15

• Routing updates are broadcast every 30 seconds by default

Routing with RIP

RIP stores information on routes in routing tables. These routing tables consist of the ultimate destination and the distance in hops (metric). RIP table entries frequently include timers associated with the route in question and a route change flag, which indicates whether information about the route has already changed.

RIP maintains routing tables in internetwork nodes. Routing tables provide the following information about each destination network the protocol knows about:

• Destination. The destination indicates the Internet Protocol (IP) address of the destination network.

• Metric. The method by which a routing algorithm determines that one route is better than another. This information is stored in routing tables. The metric value indicates the total cost of the path to the destination, in hops.

• Next hop. The next hop indicates the IP address of the next router in the path to the packets destination IP address.

• Route change flag. The route change flag indicates whether information about the route has changed recently.

• Timers. The timers values provide information about various timers associated with the path.

The following example shows part of a typical RIP routing table that does not have many of the optional RIP features activated (such as timers):

    #sho ip route rip    166.34.42.0 is variably subnetted, 42 subnets, 2 masks    R       166.34.42.168 255.255.255.255     [120/1] via 166.34.42.3, 00:00:10, Ethernet0    R       166.34.42.169 255.255.255.255     [120/1] via 166.34.42.3, 00:00:10, Ethernet0

RIP routing tables contain only the best route to a destination. They do not maintain information about multiple paths to a single destination. If a routing update provides a route with a lower metric value—that is, better—the old route is replaced.

TIPS:
It is true that RIP can support parallel paths, though detailed discussion about that aspect of its operation is beyond the scope of this book.

RIP Routing Updates

RIP sends routing update messages at regular intervals and when the network topology changes. The Cisco IOS software sends routing information updates every 30 seconds—this process is called advertising. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by the non-updating router as being unusable. This waiting time is referred to as the hold down state. If there is still no update after 240 seconds, the router removes all routing table entries for the non-updating router.

TIPS:
It should be noted that these timers are configurable, which enables a greater customization of the protocols operation within a network.

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 one, 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 independently of the regularly scheduled 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 (typically one).

When a router receives a routing update that contains a new or changed destination network entry, the router adds one 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.

Hop Count Limit

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.

The metric that RIP uses to rate the value of different routes is hop count. The hop count is the number of routers that can be traversed in a route. A directly connected network has a metric of zero; an unreachable network has a metric of 16. This small range of metrics makes RIP an unsuitable routing protocol for large networks.

If a router receives a routing update that contains a new or changed entry, and increasing the metric value by one causes the metric to be infinity (that is, more than 16 hops away), the network destination is considered unreachable.

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