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C
ONFIGURING
THE
S
WITCH
3-292
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).
A
1
3
6
4
2
5
BC
DE
A
A
B
C
D
Link Cost
0
E
1
1
3
1
0
1
2
1
2
Cost = 1 for all links
Routing table for nodeA