SMC Networks SMC6724L3 manual Configuring the Routing Information Protocol, 175

IP ROUTING

Configuring the Routing Information Protocol

The RIP protocol is the most widely used routing protocol. The RIP protocol uses a distance-vector-based approach to routing. Routes are determined on the basis of minimizing the distance vector, or hop count, which serves as a rough estimate of transmission cost. Each router broadcasts its advertisement every 30 seconds, together with any updates to its routing table. This allows all routers on the network to learn consistent tables of next hop links which lead to relevant subnets.

A		B			C		A	Link	Cost
	1		2
							A	0	0
							B	1	1
3		4
				5			C	1	2
							D	3	1
D		E					E	1	2
	6
	Cost = 1 for all links					Routing table for node A

Command Usage

•Just as Layer 2 switches use the Spanning Tree Algorithm to prevent loops, routers also use methods for preventing loops that would cause endless retransmission of data traffic. RIP utilizes the following three methods to prevent loops from occurring:

-Split horizon – Never propagate routes back to an interface port from which they have been acquired.

-Poison reverse – Propagate routes back to an interface port from which they have been acquired, but set the distance-vector metrics to infinity. (This provides faster convergence.)

-Triggered updates – Whenever a route gets changed, broadcast an update message after waiting for a short random delay, but without waiting for the periodic cycle.

•RIP-2 is a compatible upgrade to RIP. RIP-2 adds useful capabilities for plain text authentication, multiple independent RIP domains, variable length subnet masks, and multicast transmissions for route advertising (RFC 1723).

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