Components of Routing Data

Components of Routing Data
You may be thinking at this point, “What is routing and how does it work?” The “What is routing?”
part is easy to answer. Routing is the process of forwarding a packet from one place on an internetwork
to another. As for the second portion of the question, “How does it work?” that will take a
little more explanation.
The first thing you will need to understand is logical addressing. Logical addressing is used to provide
identification for each host on a network as well as for the network itself. Logical addressing
is very similar to the way addressing works for your own home. The state, city, and zip code portion
of an address is similar to the network portion of a logical address. It tells the postal service, or in
this case, the router, in what general area to find your home, or the network. Your street address,
or in this case the host address, tells the postal service, or router, exactly where you are. Upon receiving
a packet from a host, the router will need to make a routing decision. After the decision has been
made, the router will switch the packet to the appropriate interface on the router to forward it out.
You heard me right; the router actually switches packets as well as routes them.
Let’s take a look at the three obstacles a router must clear in order to make an accurate
routing decision:

Does the router that is sending and receiving the traffic know the protocol being used? The
protocols that are most widely used are IP and IPX. Other protocols, such as AppleTalk
and DECnet, may also be used.

The router then checks to see if the destination network address exists in its routing
table. The router will look for a route that matches the destination network address
with the longest matching network mask. If the router does not find a route to the
destination network, the router will discard the packet and send an ICMP destination
network unreachable message to the source of the packet.
Components of Routing Data
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A matching route must have been found or the packet will not reach this third step. From the
routing table, the router determines which interface to use to forward the packet. If the routing
table entry points to an IP address, the router will perform a recursive lookup on that next-hop
address until the router finds an interface to use. The router switches the packet to the outbound
interface’s buffer. The router then determines the layer 2 address—MAC, DLCI, and so on—
that maps to the layer 3 address. The packet is then encapsulated in a layer 2 frame appropriate
for the type of encapsulation used by the outbound interface. The outbound interface then
places the packet on the medium and forwards it to the next hop.
The packet continues this process until it reaches its destination.