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Routers can be divided into three key components: a routing engine, a forwarding engine and a management agent. The function of the routing engine is to process routing information (exchanged between routers using a routing protocols such as the Border Gateway Protocol, BGP) so as to compute routes (using a shortest path algorithms) that are stored in routing information bases (RIB) and that are composed by a destination, a next-hop interface, and a metric. Routing entries are subsequently used to populate the forwarding information base (FIB) whose entries are used by the forwarding engine. The function of the forwarding engine is to transfer incoming traffic to an outgoing interface directed towards a router closer to the traffic destination by performing a longest match prefix lookup using the incoming traffic destination address. This forwarding process is connectionless implying that at each hop the forwarding decision is taken independently for each datagram.
Most routers also have a serial connector to which a terminal (or a modem) may be connected, known as the "Console Port" (shown to the right in the figure below). This port is usually used to control the router configuration when the router is first installed. It may be the only port which is allowed to configure the filter table (used to prevent unauthorised access between the connected networks).
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