Routing Information Protocol
Routing Information Protocol uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. Cisco IOS software sends routing information updates every 30 seconds, which is termed advertising.If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by the nonupdating router as being unusable. If there is still no update after 240 seconds, the router removes all routing table entries for the nonupdating router.
A router that is running RIP can receive a default network via an update from another router that is running RIP, or the router can source (generate) the default network itself with RIP. In both cases, the default network is advertised through RIP to other RIP neighbors.
The Cisco implementation of RIP Version 2 supports plain text and Message Digest 5 (MD5) authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).
RIP Routing Updates
RIP sends routing-update messages at regular intervals and when the network topology changes. When a router receives a RIP routing update that includes changes to an entry, the router updates its routing table to reflect the new route. The metric value for the path is increased by 1, 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 RIP 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, which is typically 1. When a router receives a routing update that contains a new or changed destination network entry, the router adds 1 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. If the network of an interface network is not specified, it will not be advertised in any RIP update.
RIP Version 2 and Enabling Authentication
The Cisco implementation of RIP Version 2 supports authentication, key management, route summarization, CIDR, and VLSMs. For more information about managing authentication keys see the "Managing Authentication Keys" section of the "Configuring IP Routing Protocol-Independent Feature" module.
By default, the software receives RIP Version 1 and Version 2 packets, but sends only Version 1 packets. You can configure the software to receive and send only Version 1 packets. Alternatively, you can configure the software to receive and send only Version 2 packets. To override the default behavior, you can configure which RIP version an interface sends. Similarly, you can also control how packets received from an interface are processed.
RIP Version 1 does not support authentication. If you are sending and receiving RIP Version 2 packets, you can enable RIP authentication on an interface.
The key chain determines the set of keys that can be used on the interface. If a key chain is not configured, no authentication is performed on that interface, not even the default authentication. Therefore, you must also perform the tasks in the section "Managing Authentication Keys" in the "Configuring IP Routing Protocol-Independent Features" module.
We support two modes of authentication on an interface for which RIP authentication is enabled: plain text authentication and MD5 authentication. The default authentication in every RIP Version 2 packet is plain text authentication.