In large networks it’s almost impossible to use only static routing. The topology of these networks is changing too often, sometimes due to the extension of the network and sometimes due to link failures. To statically configure the routers every time a route changes is just impossible. Modern networks needed a routing technology able to scale to this kind of large networks. The first dynamic routing protocol ever released was RIPv1 back in 1982. However, as the networks evolved, RIP wasn’t able to scale as much as needed, so RIPv2 got his way into the networks. Of course, the time decided that modern networks needed something more scalable and we now have a larger choice of protocols: Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS), Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP), the last two being developed by Cisco. All these protocols are used as Interior Routing Protocols (IGPs), meaning that they are used for routing inside your network and will not be used for interconnecting with other networks. The today’s standard for interconnecting with other networks is the Border Gateway Protocol (BGP), or more exactly BGP4.
When using dynamic routing protocols, routers are able to fast recalculate the routes to a destination network. This process usually takes only a couple of seconds. Some protocols are able to make the changes faster while others are a little bit slower, depending on the algorithm they are using. The routers are exchanging some protocol specific messages. Depending on the protocol, these messages can be exchanged only when changes to the network occur or they can be exchanged on a regular basis.