The input ports line termination functions and data link processing implement the physical and data link layer associated with an individual input link to the router. The lookup forwarding module in the input port is central to the forwarding functions of the router. A more detailed view of input port functionality is given in figure.
In many routers the router determine the output port to which an arriving packet will be forwarded via the switching fabric. The choice of the output port is made using the information contained in the forwarding table. Although the forwarding table is typically stored at each input port and update as needed by the routing processor. With local copies of eh forwarding table the forwarding decision can be made locally at each input port without invoking the centralized routing decision can be made locally at each input port without invoking the centralized routing processor such decentralized forwarding avoids creating a forwarding processing bottleneck at a single point within the router.
Figure Input port processing
In routers with limited processing capabilities at the input port the port may simple forward the packet to the centralized routing processor when will then perform the forwarding table lookup and forward the packet to the appropriate output port.
Once the output port for a packet has been determined via the lookup the packet can be forward into the switching fabric. However a packet may be temporarily blocked from entering the switching fabric. ( due to the fact that packets from other input ports are currently using the fabric). A blocked packet must thus be queued at the input port and the scheduled to cross the switching fabric later. This process is called input port queuing. Queued datagram at the front of input queue prevents others in queue from moving forward which is block by head of the line this process is called HOL (Head of he line) blocking . input queering may delay and loss due to input buffer overflow.