How a router works

How a router works

Summary

A router is an equipment which establishes the link between various networks or sub-networks.

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Types of routers

There are several types of routers, the essential difference is that certain are normal computers equipped with network cards, and that others are equipment dedicated to the routing.

Design of a router

A router (which is dedicated) can be split in two parts:

• A backplane.
• A set of input/output ports.

The I/O

The inputs are connected to the output by the intermediary of the backplane with either:

• A shared memory (PC, bottom-of-the-range routers).
• A shared bus (medium-of-range routers).
• A point-to-point crossbar (top-of-the-range routers).

The backplane

The backplane is the motherboard which contains all the necessary components (processor, read-write memory, read-only memory) to embark a specialized operating system (for example IOS for the CISCO routers). Thus the system manages the shunting of the packets between the I/O as well as the management of the congestion.

A router can be compared to a basic computer, which is intended only for the execution of only one task: packets routing between the sender and the recipient in an optimized way.

Currently, routers are optimized according to what they must do to forward. Thus a router can exploit several points to increase his performances:

• The backplane speed
• The memories size
• Capacity of routing of the inputs.
• Capacity of service of the outputs.

Packets processing

To carry out the packets processing, a router sets up queues on the I/O, thus the input queues are used for the commutation, the routing, the classification and the output queues for scheduling, but the true heart of the router is his routing table. The routing table makes it possible to make the correspondence between the destination address and the ouput to take.

Example in the case of an IP router:

• The packet is taken on the input queue
• The packet IP address of destination is used jointly with the routing table to determine the next router (next jump) or if the destination is reached.
• The packet header is updated to register information.
• The packet is put in the selected output queue.

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