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Routing tables are generally not used directly for packet forwarding in modern router architectures; instead, they are used to generate the information for a simpler forwarding table. This forwarding table contains only the routes which are chosen by the routing algorithm as preferred routes for packet forwarding.
A forwarding information base (FIB), also known as a forwarding table or MAC table, is most commonly used in network bridging, routing, and similar functions to find the proper output network interface controller to which the input interface should forward a packet. It is a dynamic table that maps MAC addresses to ports.
IP forwarding algorithms in most routing software determine a route through a shortest path algorithm. In routers, packets arriving at an interface are examined for source and destination addressing and queued to the appropriate outgoing interface according to their destination address and a set of rules and performance metrics.
Before or after examining the destination, other tables may be consulted to make decisions to drop the packet based on other characteristics, such as the source address, the IP protocol identifier field, or Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) port number. Forwarding plane functions run in the forwarding element. [1]
The routing process usually directs forwarding on the basis of routing tables. Routing tables maintain a record of the routes to various network destinations. Routing tables may be specified by an administrator, learned by observing network traffic or built with the assistance of routing protocols. Routing, in a narrower sense of the term ...
The forwarding decision is generally made using one of two processes: routing, which uses information encoded in a device's address to infer its location on the network, or bridging, which makes no assumptions about where addresses are located and depends heavily on broadcasting to locate unknown addresses.
It does this by utilizing either a dedicated multicast routing table or, alternatively, the router's unicast routing table. When a multicast packet enters a router's interface, the router looks up the list of networks that are reachable via that interface (i.e., it checks the paths by which the packet could have arrived).
In IP-based computer networks, virtual routing and forwarding (VRF) is a technology that allows multiple instances of a routing table to co-exist within the same router at the same time. One or more logical or physical interfaces may have a VRF and these VRFs do not share routes.