Input Port

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.