In order for a router to understand where to forward packets, it must know about other inter-connected networks. A router holds details of how to reach these other networks in a ‘routing table’
Routers can have their routing tables programmed by a network administrator or they can learn these routes by discovering the topology by talking to other routers.
When routers are told by a network administrator how a network is configured and how they should forward packets, this is classed as Static Routing. The configuration entries are called static routes.
When routers, via their routing protocols, talk to each other and discover the topology of the network, this is classed as dynamic routing, this method requires less configuration and routers can re-configure routes when the topology of the network changes, such as a link going down or new links coming online.
Regardless of the method used, routers create a routing table which is a list of the routes that they know about, this table is stored in the routers RAM. There will also be metric information which sets a value as to how “attractive” a router feels the route is. Different routing protocols use different metrics. These will discussed later in the unit.
Static vs Dynamic
As static routing requires manual programming of routes, it can have a large administrative overhead when programming large inter-networks as it can take a large amount of effort to keep all routers up to date. Also when using static routing, If a node is faulty, is newly added or goes down, the tables will have to be altered manually. This method does not cope well with unplanned events such as breakdown or congestion.
As this information is static and the routers do not continually have to talk to each other and keep up to date with the latest topology, the static method does use less router processor overhead and therefore is faster.
Most routers are dynamic with the capability of being statically configured.