RIP

RIP, a UDP-based protocol, updates its neighbors, and the neighbors update their neighbors, and so on. Each RIP host 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.

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.

• Source IP address

  The Link-Local IPv6 address of the interface sending the RIPng packet is used as the source IP address of any RIPng update sent.

• Destination IP address

  The destination IP for any periodic or triggered update should be sent to the multicast group FF02::9, (all-rip-routers multicast group). When sending responses to an RIPng request, the RIPng response is sent to the unicast IP address of the requester.

Each route entry in an update message contains the following:

• IPv6 prefix

• prefix length

• route metric

• route tag (optional)

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 A metric value of 16 (infinity) indicates that the route is no longer valid and should be removed from the router’s routing table.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 one.

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 one 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 specified interface are advertised in updates out of the same interface but with a metric of 16 (infinity).

RIP uses the following timers to determine the frequency of RIP updates and the duration that routes are maintained.

• update

  Times the interval between periodic routing updates.

• timeout

  This timer is initialized when a route is established and any time an update message is received for the route. When this timer expires, the route is no longer valid. It is retained in the table for a short time, so that neighbors can be notified that the route has been dropped.

• flush

  When the flush timer expires, the route is removed from the tables.

Routing policies can control the content of the routing tables, advertised routes, and the best route 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 learned routes to its neighbors.

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

The minimum RIP configuration must define one group and one neighbor. For more information about RIP hierarchy levels, see Basic RIP configuration.

Before RIP neighbor command options 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.

This section provides information to configure RIP and examples of common configuration tasks. For a router to accept RIP updates, in the configure router rip context, you must define at least one group and one neighbor. A router ignores updates received from routers on interfaces not configured for RIP. Configuring other RIP commands and command options is optional.

By default, the local router imports all routes from this neighbor and does not advertise routes. The router receives both RIPv1 and RIPv2 update messages with 25 to 255 route entries per message.

The RIP configuration commands have three primary configuration levels:

• rip for RIP global configurations
• group for RIP group configurations
• neighbor for RIP neighbor configurations

Within these levels, the RIP configuration commands are identical. For repeated commands, the value most specific to the neighboring router is used. Therefore, a RIP group-specific command takes precedence over a global RIP command. A neighbor-specific configuration statement takes precedence over a global RIP and group-specific command. For example, if the user modifies a RIP neighbor-level command default, the new value takes precedence over group- and global-level settings.

At a minimum, the group- and neighbor-level RIP command options must be configured in the configure router rip context.

The following example displays a basic RIP configuration.

[ex:/configure router "Base" rip]
A:admin@node-2# info
    group "RIP-ALA-A" {
        neighbor "to-ALA-4"
    }

A:node-2>config>router>rip# info
----------------------------------------------
            group "RIP-ALA-A"
                neighbor "to-ALA-4"
                    no shutdown
----------------------------------------------

Modify, add, or remove RIP command options in the CLI. The changes are applied immediately. For the complete list of CLI commands, see the Configuring RIP command options.

In the classic CLI, you must administratively disable a group before deleting it using the following command.

configure router rip group shutdown

INFO: RIP #1204 group should be administratively down - virtual router index 1,group
RIP-ALA-4 

In the classic CLI, you must administratively disable a neighbor before deleting it using the following command.

configure router rip group neighbor shutdown

Deleting the neighbor without first shutting it down causes the following message to appear.

INFO: RIP #1101 neighbor should be administratively down - virtual router index