Planet WGSW-404 User Manual page 92

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. 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 1388).
There are several serious problems with RIP that you should consider before deciding which routing
protocol to use for your network. 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, nor does it consider enough network variables to make the best routing
decision.
Note: Before you can define a VLAN's IP settings, you must first create a VLAN as described under
"Creating a New VLAN".
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