DYNAMIC ROUTING

 The chief advantages of dynamic routing over static routing are scalability and adaptability. A dynamically routed network can grow more quickly and larger, and is able to adapt to changes in the network topology brought about by this growth or by the failure of one or more network components.

With a dynamic routing protocol, routers learn about the network topology by communicating with other routers. Each router announces its presence, and the routes it has available, to the other routers on the network. Therefore, if you add a new router, or add an additional segment to an existing router, the other routers will hear about the addition and adjust their routing tables accordingly. You don't have to reconfigure the routers to tell them that the network has changed. Similarly, if you move a network segment, the other routers will hear about the change. You only need to change the configuration of the router (or routers) that connect the segment that moved. This reduces the chance that errors will occur.

The ability to learn about changes to the network's configuration has implications beyond adding new segments or moving old ones. It also means that the network can adjust to failures. If a network has redundant paths, then a partial network failure appears to the routers as if some segments got moved (they are now reached via alternate paths), and some segments have been removed from the network (they are now unreachable). In short, there's no real difference between a network failure and a configuration change. Dynamic routing allows the network to continue functioning, perhaps in a degraded fashion, when a partial failure occurs.

Disadvantages of Dynamic Routing

   I would be a liar if I told you that dynamic routing has no disadvantages. Chief among the disadvantages is an increase in complexity. Communicating information about network topology is not as simple as saying, "Hey, I can reach the following destinations..." Each router participating in the dynamic routing protocol must decide exactly what information to send to other routers; more important, it must attempt to select the best route for reaching other destinations from the candidates it learns about from other routers. In addition, if a router is going to adapt to changes in the network, it must be prepared to remove old or unusable information from its routing table. How it determines what is old or unusable adds to the complexity of the routing protocol. Unfortunately, the better a protocol handles the various different situations in a network, the more complex it is likely to be. This complexity tends to lead to errors in the protocol's implementation, or differences in how vendors interpret the protocol.

In order to communicate information about the topology of the network, routers must periodically send messages to each other using a dynamic routing protocol. These messages must be sent across network segments just like any other packets. But unlike other packets in the network, these packets do not contain any information to or from a user. Instead, they contain information that is only useful to the routers. Thus, from the users' point of view, these packets are pure overhead. On a low-speed link, these messages can consume much of the available bandwidth, especially if the network is large or unstable.

Finally, some or all of the machines in a network may be unable to speak any dynamic routing protocol, or they may not speak a common protocol. If that is the case, static routing may be your only option.

With all the disadvantages listed of both static and dynamic routing, you may be wondering what the best choice is. Only you can say for sure what is best for your network, but there is a reasonable middle ground that limits the complexity of dynamic routing without sacrificing its scalability. This middle ground is a hybrid scheme, in which part of the network uses static routing and part uses dynamic routing.