For routing between routers to work efficiently in an internetwork, routers must have knowledge of other network IDs or be configured with a default route. On large internetworks, the routing tables must be maintained so that the traffic always travels along optimal paths. How the routing tables are maintained defines the distinction between static and dynamic routing.
A router with manually configured routing tables is known as a static router. A network administrator, with knowledge of the internetwork topology, manually builds and updates the routing table, programming all routes in the routing table. Static routers can work well for small internetworks but do not scale well to large or dynamically changing internetworks due to their manual administration.
Static routers are not fault tolerant. The lifetime of a manually configured static route is infinite and, therefore, static routers do not sense and recover from downed routers or downed links.
A good example of a static router is a multihomed computer running Windows 2000 (a computer with multiple network interface cards). Creating a static IP router with Windows 2000 is as simple as installing multiple network interface cards, configuring TCP/IP, and enabling IP routing.
A router with dynamically configured routing tables is known as a dynamic router. Dynamic routing consists of routing tables that are built and maintained automatically through an ongoing communication between routers. This communication is facilitated by a routing protocol, a series of periodic or on-demand messages containing routing information that is exchanged between routers. Except for their initial configuration, dynamic routers require little ongoing maintenance, and therefore can scale to larger internetworks.
Dynamic routing is fault tolerant. Dynamic routes learned from other routers have a finite lifetime. If a router or link goes down, the routers sense the change in the internetwork topology through the expiration of the lifetime of the learned route in the routing table. This change can then be propagated to other routers so that all the routers on the internetwork become aware of the new internetwork topology.
The ability to scale and recover from internetwork faults makes dynamic routing the better choice for medium, large, and very large internetworks.
A good example of a dynamic router is a computer with Windows 2000 Server and the Routing and Remote Access Service running the Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) routing protocols for IP and RIP for IPX.