Dynamic and Static Routing

Entries are imported into a router's routing table from dynamic routing protocols or by manual inclusion as static routes. Dynamic routing protocols allow routers to learn the network topology from the network. The routers within the network send out routing information in the form of route advertisements. These advertisements establish and communicate active destinations, which are then shared with other routers in the network.

Although dynamic routing protocols are extremely useful, they have associated costs. Because they use the network to advertise routes, dynamic routing protocols consume bandwidth. Additionally, because they rely on the transmission and receipt of route advertisements to build a routing table, dynamic routing protocols create a delay (latency) between the time a router is powered on and the time during which routes are imported into the routing table. Some routes are therefore effectively unavailable until the routing table is completely updated, when the router first comes online or when routes change within the network (due to a host going offline, for example).

Static routing avoids the bandwidth cost and route import latency of dynamic routing. Static routes are manually included in the routing table, and never change unless you explicitly update them. Static routes are automatically imported into the routing table when a router first comes online. Additionally, all traffic destined for a static address is routed through the same router. This feature is particularly useful for networks with customers whose traffic must always flow through the same routers. Figure 38 shows a network that uses static routes.

In Figure 38, the customer routes in the 192.176.14/24 subnetwork are static routes. These are hard links to specific customer hosts that never change. Because all traffic destined for any of these routes is forwarded through router A, these routes are included as static routes in router A's routing table. Router A then advertises these routes to other hosts so that traffic can be routed to and from them.