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 retransmissio n o f
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 interfac e port from whi ch 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 sh ould cons ider. First of all ,
RIP (version 1) has no knowledge of subnets, bot h RIP vers ions can tak e 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 restr icts its u se to
smaller networks. Moreover, RIP (version 1) wastes valuable network bandwidth
by propagating routing information via broadcasts; it als o consid ers too fe w
network variables to make the best routing decision.

20-2

Configuring the Routing Information Protocol
The RIP protocol is the mos t wide ly u sed ro utin g protoc ol. T he RIP prot ocol u ses a
distance-vector-based approach to routing. Routes are determin ed on the bas is of
minimizing the distance vector, or hop count, which serves as a rough estim ate of
transmission cost. Each router broadcasts its advertisement every 30 second s,
together with any updates to its routing table. This allows al l routers on the net work
to learn consistent tables of next hop links which lead to relevant subnets.
A
A
B
C
D
E
Link Cost
0 0
1 1
1 2
3 1
1 2
A
3
D
B
4
E
C
5
1
6
2
Unicast Routing

20