Router components - network layer and routing , Computer Networking

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Router Components

As noted in the  previous  a router consists of a of input  interfaces at which  packets arrive and a set  of output interfaces from  which  packets depart. The  input and output interfaces are interconnected  by a high  sped fabric that allows packets to be transferred from  inputs  to outputs. The main  parameter that characterizes the fabric is the speed  up. The speedup is defined as the  ration between  the maximum transfer rate across the fabric from an  input  to an output interface and the capacity of an input link.

The set of operation performed by routers on a data packet as the packet travels from  source to destination. The  main functions performed by routers on the data path are routing lookup buffer management and  packet scheduling routing lookup identifies the output interface where to forward each incoming  packet based on the  destination address in the packet heard. Buffer  management and scheduling are concerned with managing  router  resources in case of congestion. In particular when the  buffer overflow or when it exceeds some predefined threshold the router  has to  decide what packet to drop. Similarly when there is more than one packet in the buffer the  router has to decide what packet to transmit next. Usually today routers implement a simple drop tail  buffer management scheme that is  when the buffer overflow the  packet at the tail of the queue is dropped  packets are scheduled on a first in first out  basis.

As  a packet  traverse a router the packet  can be stored at input  at output  or at both  the input  and  output  interfaces. Based  on where a router can stores  packets routers are classified as input queuing output queuing or input output queuing.

A high level  view of  a generic router  architecture is shown  in figure four  components  of a router can be identified.

Input port  performs  the physical  layer functions  of terminating and incoming physical  link to  a router. It also performs the data link layer function  that middle boxes in the input and out port needed to inter operate with the data link layer functions  at the remote side of the incoming links. It also performs a lookup  and forwarding function  so that a packet  forwarded into the  switching  fabric of the router  emerge at the appropriate output  port. Control  packets  are forwarded  from an input  port to the routing  processor.

Output port stores the  packets  that have  been forwarded  to it through the switching  fabric  and then  transmits the packets on the outgoing link. The output port thus  performs  the reverse data link and physical  layer functionality of the output  port. When a link  is bidirectional an to the link  will typically by paired  with the  input  port  for that  same link  card.

Routing  process executes the routing protocols  maintains the routing  information and forwarding  tables and performs  networks  management  function within  the router.

Switching fabric  connects the  router  input  ports to tits output  ports. This switching  fabric  is completely contained within  the router  a network  inside a network  router.


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