RFC 1058 from June 1988 defines RIPv1. RIP is a distance-vector routing protocol that uses router hop count as the metric. RIP is a classful routing protocol that does not support VLSMs or CIDR.
There is no method for authenticating route updates. A RIP router sends a copy of its routing table to its neighbors every 30 seconds. RIP uses split horizon with poison reverse; therefore, route updates are sent out an interface with an infinite metric for routes learned (received) from the same interface.
The RIP standard was based on the popular routed program used in UNIX systems since the 1980s. The Cisco implementation of RIP adds support for load balancing. RIP will load-balance traffic if there are several paths with the same metric (equal-cost load balancing) to a destination. Also, RIP sends triggered updates when the metric of a route changes. Triggered updates can help the network converge faster rather than wait for the periodic update. RIP has an administrative distance of 120. Chapter 11, “Routing Protocol Selection Criteria,” covers administrative distance.
RIP summarizes to IP network values at network boundaries. A network boundary occurs at a router that has one or more interfaces that do not participate in the specified IP network. The IP address assigned to the interface determines participation. IP class determines the network value. For example, an IP network that uses 24-bit subnetworks from 220.127.116.11/24 to 18.104.22.168/24 is summarized to 22.214.171.124/16 at a network boundary.
RIPv1 Forwarding Information Base
The RIPv1 protocol keeps the following information about each destination:
- IP address—IP address of the destination host or network
- Gateway—The first gateway along the path to the destination
- Interface—The physical network that must be used to reach the destination
- Metric—A number indicating the number of hops to the destination
- Timer—The amount of time since the entry was last updated
The database is updated with the route updates received from neighboring routers. As shown in Example 12-1, the show ip rip database command shows the RIP private database of a router.
Example 12-1 show ip rip database Command
router9# show ip rip database 172.16.0.0/16 auto-summary 172.16.1.0/24 directly connected, Ethernet0 172.16.2.0/24  via 172.16.4.2, 00:00:06, Serial0 172.16.3.0/24  via 172.16.1.2, 00:00:02, Ethernet0 172.16.4.0/24 directly connected, Serial0 Read more...