Levelone GTL-2690 User Manual download pdf (Page 343)

100

101

Unicast Routing

20-2

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.

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).

• There are several serious problems with RIP that you should consider. First of all,

RIP (version 1) has no knowledge of subnets, both RIP versions can take a long

time to converge on a new route after the failure of a link or router during which time

routing loops may occur, and its small hop count limitation of 15 restricts its use to

smaller networks. Moreover, RIP (version 1) wastes valuable network bandwidth

by propagating routing information via broadcasts; it also considers too few

network variables to make the best routing decision.

Link Cost

Cost = 1 for all links

Routing table for node A

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Levelone GTL-2690 User Manual Page 343

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