Static Versus Dynamic Route Assignment
Static routes are manually configured on a router. They do not react to network outages. The one exception is when the static route specifies the outbound interface: If the interface goes down, the static route is removed from the routing table. Because static routes are unidirectional, they must be configured for each outgoing interface the router will use. The size of today's networks makes it impossible to manually configure and maintain all the routes in all the routers in a timely manner. Human configuration can involve many mistakes, which is why routing protocols exist. They use algorithms to advertise and learn about changes in the network topology.
The main benefit of static routing is that a router generates no routing protocol overhead. Because no routing protocol is enabled, no bandwidth is consumed by route advertisements between network devices. Another benefit of static routing protocols is that they are easier to configure and troubleshoot than dynamic routing protocols. Static routing is recommended for hub-and-spoke topologies with a low-speed remote connection. A default static route is configured at each remote site because the hub is the only route used to reach all other sites. Static routers are also used at network boundaries (Internet or partners) where routing information is not exchanged. These static routes are then redistributed into the internal dynamic routing protocol used.
Figure 9-1 shows a hub-and-spoke WAN where static routes are defined in the remote WAN routers because no routing protocols are configured. This setup eliminates routing protocol traffic on the low-bandwidth WAN circuits.
Figure 9-1 Static Routes in a Hub-and-Spoke Network
Routing protocols dynamically determine the best route to a destination. When the network topology changes, the routing protocol adjusts the routes without administrative intervention. Routing protocols use a metric to determine the best path toward a destination network. Some use a single measured value such as hop count. Others compute a metric value using one or more parameters. Routing metrics are discussed later in this chapter. The following is a list of dynamic routing protocols:
Figure 9-1 Static Routes in a Hub-and-Spoke Network
■ Border Gateway Protocol (BGP)
Continue reading here: Distance Vector Routing Protocols
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