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Routing Basics

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Routing Basics

nce you create an internetwork by connecting your WANs and

LANs to a router, you then need to configure logical network addresses, such
as IP addresses, to all hosts on the internetwork so that they can communi-
cate across the internetwork.

The term

routing

is used for taking a packet from one device and sending

it through the network to another device on a different network. Routers don't
care about hosts--they only care about networks and the best path to each
network. The logical network address of the destination host is used to get
packets to a network through a routed network, then the hardware address of
the host is used to deliver the packet from a router to the destination host.

If your network has no routers, then you are not routing. (Makes sense,

huh?) Routers route traffic to all the networks in your internetwork. To be
able to route packets, a router must know, at a minimum, the following:

Destination address

Neighbor routers from which it can learn about remote networks

Possible routes to all remote networks

The best route to each remote network

How to maintain and verify routing information

The router learns about remote networks from neighbor routers or from

an administrator. The router then builds a routing table that describes how to
find the remote networks. If a network is directly connected, then the router
already knows how to get to it. If a network is not connected, the router must
learn how to get to the remote network using either static routing, which
means that the administrator must hand-type all network locations into the
routing table, or dynamic routing.

dynamic routing

, a protocol on one router communicates with the

same protocol running on neighbor routers. The routers then update each
other about all the networks they know about and place this information in
the routing table. If a change occurs in the network, the dynamic routing
protocols automatically inform all routers about the change. If

static routing

is used, the administrator is responsible for updating all changes by hand
into all routers. Typically, in a large network, a combination of both
dynamic and static routing is used.

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