8.1 Autonomous Systems
8.1.2 Static routing, default routing and dynamic routing
Before looking at the basic ways in which ASs can be connected to Internet service providers (ISPs), you need to review basic routing terminology and concepts.
  • Static routing refers to routes, which are manually configured in the router. With manually configured routes, network reachability is not dependent on the state of the network -- the router trusts the human operator to make correct routing decisions. Whether a destination is up or down, static routes remain in the routing table, and traffic is still forwarded based on them.
  • Default routing uses a "last resort" route. If a router cannot find a packet's destination network in its table, it will use a default route to forward the traffic. Default routing is the easiest form of routing for a domain connected to a single exit point.
  • Dynamic routing refers to routes learned via routing protocols. With dynamic routing, network reachability is dependent on the state of the network. If a destination is down, the route will disappear from the routing table, and traffic will not be sent toward it.

You will find that static, dynamic, and default routing each have their place in a network architecture; but one method (or combination of methods) will prove optimal depending on the situation. This chapter considers several typical architectures and the optimal routing method for each. 

Static routes and default routes are not your enemy! Although static routes don't provide much flexibility, they can be the most stable configurations. Many people think they are not technologically up-to-date just because they are not running dynamic routing -- but trying to force dynamic routing on situation that does not really need it is just a waste of bandwidth, effort, and money.