A Made IT project
According to the OSI-model the router doesn't exist, but is called a gateway. Since the meaning of a gateway in everyday speach is different, we will use the term router to address a device that corresponds to the following features:
A router connects networks, thus working on the network layer of the OSI-model (Layer 3). It is protocol depended and the network address on one interface is different from that on another interface.
The most common application for routers is to connect LANs through WAN-links.
Routers are used to connect LANs or coupled LANs. Their means is to provide a controlled way for the delivery of packets over WAN-links and to separate LANs and/or protocols. Since a router functions on layer 3 of the OSI model, it depends on routable protocols (not all protocols are routable). The protocols need to have a way of having network numbers on which a router can base its descision to route a packet.
Another advantage of routers is that different paths can be provided to one target and the most efficient (read fastest) path can be choosen to deliver a message. To do this, routers needs to know what the network around them look like.
Through this path selection method a router can provide redundant paths to one location. This way a link-failure will not stop the connectivity. A router will make sure the damaged link is not used anymore and a different path is chosen to send the packets.
How a router works
Since routers are working on layer 3 of the OSI model they are capable of analyzing a packet up til the network address-envelope. Let's assume that a packet looks like this:
The router strips the MAC-address and reads the destination-address. It then examens this address and decides if it is on a network on one of its local ports. If so it attaches the MAC-address of the station to the packet and sends it to the port where the station is on.
If it is not a station on one of its local attached networks it will look at the network part of the destination-address. And chooses the port to send it out on. The packet will be send to another router and the MAC-address of that router will be attached to the packet. The next router does the same and eventually the packet will reach its destination.
In basic this is what routers do. In everyday life routers need to know what port should be used for which network and what the alternative routes are to reach that network and which stations are on which port and so on. To know all this routers need to have a way to communicate to eachother. And since there are a lot of different vendors, there need to be a standard to do this so every router can be connected to any other router.
To make a first distinction: you can split routers into two different kinds. One type uses routing tables and one type does not. To look at the non-routing-table routers first: You have three different types:
All these techniques are old, and are rarely seen anymore. Most modern routers use routing-tables to make sure the packets will arrive save and as fast as possible. These routing-table routers are there in two flavours and most of the time the two techniques are comined:
|protocol||Distance Vector||Link State|
|TCP IP||RIP, IGRIP||OSPF|
|DECnet||DECnet phase IV||DECnet phase V|
There are a lot more protocols but these are the most important ones. click on their names to learn more about them.
Distance Vector is also known as Bellman-Ford. The routers using this technique send their complete routing table to their neighbour routers. This can be an enormous amount network traffic.
Also known as Shortes Path First sends only updates to the neighbour routers. This generates less traffic, but it uses more computation power on the router.