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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 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.
4.2.6.2.OSPFv2 Dynamic Routing Protocol
OSPF overcomes all the problems of RIP. It uses a link state routing protocol to
generate a shortest-path tree, then builds up its routing table based on this tree. OSPF
produces a more stable network because the participating routers act on network
changes predictably and simultaneously, converging on the best route more quickly than
RIP. Moreover, when several equal-cost routes to a destination exist, traffic can be
distributed equally among them.
OSPF looks at more than just the simple hop count. When adding the shortest path to
any node into the tree, the optimal path is chosen on the basis of delay,
throughput and connectivity. OSPF utilizes IP multicast to reduce the amount of routing
traffic required when sending or receiving routing path updates. The separate routing
area scheme used by OSPF further reduces the amount of routing traffic, and thus
inherently provides another level of routing protection. In addition, all routing protocol
exchanges can be authenticated. Finally, the OSPF algorithms have been tailored for
efficient operation in TCP / IP Internets.
