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Chapter 1
Concepts and terminology
Anatomy of a basic router
The picture below shows the anatomy of a basic router, consisting of input/output ports, IP interfaces, a
forwarding table and a forwarding process.
Input Communication Link
>
Input/ output communication links
These are the real communication media, such as Ethernet or ATM virtual channels mapped on a DSL
line. Communication links are attached to the router's external communication ports.
>
IP interfaces (input - output)
Similar as Port, an Interface is a host's or router's attachment point to a communication medium or link.
>
Forwarding process
A router's forwarding process performs the actual IP packet forwarding. That is, for every packet
presented at an input port, the forwarding process performs a lookup in the forwarding table to
determine the output port on which to direct the packet.
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Forwarding table
The forwarding table contains all destination networks of which the router is aware. The forwarding
process relies on the contents of this table for all its decisions regarding IP packet forwarding.
E-DOC-CTC-20051017-0168 v1.0
IP Packet
Input IP Interface
Port 1
Destination IP
Destination network
Destination network a
Destination network b
...
Destination network n
Router
Forwarding Process
Forwarding action
Output IP Interface
Port 2
Port
router
port
Port 1
Port 2
...
Port x
Forwarding
Table
Output Communication Link
5
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