2.2.1. RIP Features

RIP, a UDP-based protocol, updates its neighbors, and the neighbors update their neighbors, and so on. Each host that uses RIP has a routing process that sends and receives datagrams on UDP port number 520.

Each RIP router advertises all RIP routes periodically via RIP updates. Each update can contain a maximum of 25 route advertisements. This limit is imposed by RIP specifications. RIP can sometimes be configured to send as many as 255 routes per update. The formats of the RIPv1 and RIPv2 updates are slightly different and are shown below. Additionally, RIPv1 updates are sent to a broadcast address, RIPv2 updates can be either sent to a broadcast or multicast address (224.0.0.9). RIPv2 supports subnet masks, a feature that was not available in RIPv1.

A network address of 0.0.0.0 is considered a default route. A default route is used when it is not convenient to list every possible network in the RIP updates, and when one or more closely-connected gateways in the system are prepared to handle traffic to the networks that are not listed explicitly. These gateways create RIP entries for the address 0.0.0.0, as if it were a network to which they are connected.

2.2.1.3. RIP Packet Format

The RIP packet format is displayed in Figure 1:

Figure 1:  RIP Packet Format 

A RIP packet consists of the following fields:

1. Command — Indicates whether the packet is a request or a response message. The request asks the responding system to send all or part of its routing table. The response may be sent in response to a request, or it may be an unsolicited routing update generated by the sender.
2. Version — The RIP version used. This field can signal different potentially incompatible versions.
3. Must be zero — Not used in RIPv1. This field provides backward compatibility with pre-standard varieties of RIP. The default value is zero.
4. Address family identifier (AFI) — The AFI is the type of address. RIP can carry routing information for several different protocols. Each entry in this field has an AFI to indicate the type of address being specified. The IP AFI is 2.
5. Address — The IP address for the packet.
6. Metric — Specifies the number of hops to the destination.
7. Mask — Specifies the IP address mask.
8. Next hop — Specifies the IP address of the next router along the path to the destination.

2.2.1.3.1. RIPv1 Format

There can be between 1 and 25 (inclusive) RIP entries. Figure 2 displays RIPv1 format:

Figure 2:  RIPv1 Format 

2.2.1.3.2. RIPv2 Format

The RIP packet format is displayed in Figure 3:

Figure 3:  RIPv2 Format 

The RIPv2 packets include the following fields:

1. Subnet mask — The subnet mask for the entry. If this field is zero, no subnet mask has been specified for the entry.
2. Next hop —The IP address of the next hop to forward packets.

2.3.1. RIPng Protocol

RIPng packets are sent using the UDP protocol and the protocol port number 521. Unsolicited updates messages are sent with 521 as both the source and destination port.

Each route entry in an update message contains the following:

2.4.1. Metrics

By default, RIP advertises all RIP routes to each peer every 30 seconds. RIP uses a hop count metric to determine the distance between the packet’s source and destination. The metric/cost values for a valid route is 1 through 15. A metric value of 16 (infinity) indicates that the route is no longer valid and should be removed from the router’s routing table.

Each router along the path increments the hop count value by 1. When a router receives a routing update with new or different destination information, the metric increments by 1.

The maximum number of hops in a path is 15. If a router receives a routing update with a metric of 15 and contains a new or modified entry, increasing the metric value by 1 will cause the metric increment to 16 (infinity). Then, the destination is considered unreachable.

The router implementation of RIP uses split horizon with poison reverse to protect from such problems as “counting to infinity”. Split horizon with poison reverse means that routes learned from a neighbor through a given interface are advertised in updates out of the same interface but with a metric of 16 (infinity).

2.4.2. Timers

RIP uses numerous timers to determine how often RIP updates are sent and how long routes are maintained.

2.4.3. Import and Export Policies

Routing policies can control the content of the routing tables, the routes that are advertised and the best route to take to reach a destination. Import route policies determine which routes are accepted from RIP neighbors. Export route policies determine which routes are exported from the route table to RIP. By default, RIP does not export routes it has learned to its neighbors.

There are no default routing policies. A policy must be created explicitly and applied to a RIP import or export command.

2.4.4. Hierarchical Levels

The minimum RIP configuration must define one group and one neighbor. The parameters configured on the global level are inherited by the group and neighbor levels. Parameters can be modified and overridden on a level-specific basis. RIP command hierarchy consists of three levels:

Many of the hierarchical RIP commands can be modified on different levels. The most specific value is used. That is, a RIP group-specific command takes precedence over a global RIP command. A neighbor-specific statement takes precedence over a global RIP and group-specific command; for example, if you modify a RIP neighbor-level command default, the new value takes precedence over group- and global-level settings.

2.6.1. General

Before RIP neighbor parameters can be configured, router interfaces must be configured.

RIP must be explicitly created for each router interface. There are no default RIP instances on a router.