The objective of this lesson is to explain routing. We’ll
start by first defining what routing is. We’ll follow that with a discussion
on addressing. There is a section on routing terminology which covers subjects
like routed vs. routing protocols and dynamic and static routing. Finally, we’ll
talk about routing protocols.
- What Is Routing?
- LAN-to-LAN Connectivity
- Routing Tables and
- Network Addressing
- Static Routing and
- Routing Protocols
What Is Routing?
Routing is the process of finding a path to a destination host
and of moving information across an internetwork from a source to a destination.
Along the way, at least one intermediate node typically is encountered. Routing
is very complex in large networks because of the many potential intermediate destinations
a packet might traverse before reaching its destination host.
A router is a device that forwards packets from one network to another and determines
the optimal path along which network traffic should be forwarded. Routers forward
packets from one network to another based on network layer information. Routers
are occasionally called gateways (although this definition of gateway is becoming
A router is a more sophisticated device than a hub or a switch..
It determines the appropriate network path to send the packet along by keeping
an up-to-date network topology in memory, its routing table.
A router keeps a table of network addresses and knows which path to take to get
to each network.
Routers keep track of each other’s routes by alternately listening, and
periodically sending, route information. When a router hears a routing update,
it updates its routing table. Routing is often contrasted with bridging, which
might seem to accomplish precisely the same thing to the causal observer. The
primary difference between the two is that bridging occurs at Layer 2 (the data
link layer) of the OSI reference model, whereas routing occurs at Layer 3 (the
network layer). This distinction provides routing and bridging with different
information to use in the process of moving information from source to destination,
so that the two functions accomplish their tasks in different ways.
In addition, bridges can’t block a broadcast (where a data packet is sent
to all nodes on a network). Broadcasts can consume a great deal of bandwidth.
Routers are able to block broadcasts, so they provide security and assist in bandwidth
You might ask, if bridging is faster than routing, why do companies move from
a bridged/switched network to a routed network?
There are many reasons, but LAN segmentation is a key reason. Also, routers increase
scalability and control broadcast transmissions.
Where are Routers Used?
A router can perform LAN-to-LAN routing through its ability
to route packet traffic from one network to another. It checks its router table
entries to determine the best path to the destination network.
A router can perform LAN-to-WAN and remote access routing through its ability
to route packet traffic from one network to another while handling different WAN
services in between. Popular WAN service options include Integrated Services Digital
Network, or ISDN, leased lines, Frame Relay, and X.25.
Let’s look at routing in more detail.