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.