10.1.1 Overview

The Routing Information Protocol (RIP) is used between the routers connected on a network. By using RIP, each router generates its own routing information by exchanging information about the networks it can reach and the number of hops (metrics) to those networks.

The Switch supports RIP versions 1 and 2. RIP version 0 messages are discarded. Messages generated from RIP version 3 or higher are handled as version 2 messages.

The following table describes the functionality supported by RIP.

Table 10-1 RIP features
Functionality	RIP
Triggered update	OK
Split horizon	OK
Route poisoning	OK
Poison reverse	NG
Hold-downs	NG
RIP automatic route summarization	OK
Route tagging	OK
Read specified next hop	OK
Plain-text password authentication	OK
Cryptographic authentication (Keyed-MD5)	OK

Legend: OK: Supported, NG: Not supported

<Structure of this section>

(1) Message Type

RIP uses two types of messages: request messages and response messages. A request message is used to request routing information from another router. A response message is used to reply to a request from another router. Response messages are also used to notify other routers of Switch's routing information, either periodically or when there is a change in the network topology.

(2) Operation processing

At startup, the Switch sends all of its request messages to neighboring routers, and requests neighboring routers to send all of their routing information. After startup, the Switch sends responses for the following purposes:

To send relevant routing information in response to a request from a neighboring router
To periodically report routing information. At 30-second intervals the Switch sends all of its routing information as responses to neighboring routers.
To report a detected route change. The Switch sends routing information regarding a changed route as a response to neighboring routers.

On receipt of a response from a neighboring router, the Switch updates its routing information if a route change has been detected. Responses are also used to check the transmission status to a neighboring router. If no response is received for 180 seconds or longer, the neighboring router is deemed unreachable and the routing table is updated with an alternate route, if available. Otherwise, the unreachable route is deleted.

(3) Suppressing Routing Loops

The Switch use split horizon logic to prevent loops in forwarding routes. Split horizon processing stops received information from being forwarded to the interface from which the information originated.