What is Forwarding plane?

In routing, the forwarding plane, sometimes called the data plane or user plane, defines the part of the router architecture that decides what to do with packets arriving on an inbound interface. Most commonly, it refers to a table in which the router looks up the destination address of the incoming packet and retrieves the information necessary to determine the path from the receiving element, through the internal forwarding fabric of the router, and to the proper outgoing interface.

In certain cases the table may specify that a packet is to be discarded. In such cases, the router may return an ICMP "destination unreachable" or other appropriate code. Some security policies, however, dictate that the router should drop the packet silently, in order that a potential attacker does not become aware that a target is being protected.

The incoming forwarding element will also decrement the time-to-live field of the packet, and, if the new value is zero, discard the packet. While the Internet Protocol specification indicates that an Internet Control Message Protocol time exceeded message should be sent to the originator of the packet , the router may be configured to drop the packet silently.

Depending on the specific router implementation, the table in which the destination address is looked up could be the routing table , or a separate forwarding information base that is populated by the routing control plane, but used by the forwarding plane for look-ups at much higher speeds. 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 or User Datagram Protocol port number.