Discover how routers power internetworks
Discover how routers
power internetworks
Routers have been around commercially since the late
1980s. They were not really prevalent until the early 1990s and have
increased their position in the internetwork directly because of the
Internet. It is important to understand that routers, in some fashion, will
never go away.
What is a router?
Routers are hardware devices that use software to perform routing of
packets in an internetwork. Routing is the term used to define the process
of taking a packet of data from a device on one network and sending it
through the router to another device on a different network. If your network
has no routers, then you are not routing. Routers are used within a network
to route traffic to all the networks in your internetwork.
In Figure A, notice how both LANs and WANs are connected to a router.
This is the main purpose of a router—to break up broadcast domains. By
connecting multiple networks to a router, you create an internetwork. The
E0, E1, etc., are 10Base-T Ethernet links, and the Fa interfaces are Fast
Ethernet connections.
Figure A |
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Notice how both LANs and WANs are connected
to a router. |
Once you connect networks to a router, you must provide logical addressing
to each device so it can communicate on the internetwork. Which brings up a
very important point: You must be able to uniquely identify every device on
the internetwork, regardless of where these devices are located. This is
called logical addressing.
Let's define the difference between a logical address
and a hardware address:
- A logical address uniquely identifies a device on
an internetwork.
- A hardware (MAC) address uniquely identifies a
device on a LAN.
Figure B shows how a device communicates on a
LAN using the hardware address of the device and how the same device
communicates to another device on the internetwork using a logical address.
Figure B |
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Devices communicate on a LAN using the
hardware address of the device. |
The benefit of routers
Unlike switches (bridges), routers by default break up broadcast
domains. This is a good thing, if the network is designed correctly. (I'll
further explain domains in the next section of this article.)
Routers provide security by filtering the network. By placing routers in
optimal positions in your network, you can effectively allow and deny
packets from being transmitted all over the internetwork. These filters are
called access lists.
Routers also provide connection to WAN services. Although many switches
allow a card to be placed in the switch to provide this service, this is a
Layer 3 technology, and a router or Layer 3 device is needed to provide WAN
connection services.
Routers break up broadcast domains
In a previous article, I also discussed collision domains and how
switches break up collision domains by default. However, switches create one
large broadcast domain.
A collision domain is defined as a network segment that shares bandwidth
with every device connected to this particular segment. If one device
transmits, all other devices on this segment must listen and not transmit.
If a second device does transmit at the same time, a collision occurs. By
using switches, we can create individual collision domains, but if one host
sends any type of broadcast, all segments connected to the switch must
listen. To break up broadcast domains in a Layer 2 switched internetwork,
you have two options: routers or Virtual LANs (VLANs).
Routers plug in to a hub or switch port, and every device connected to that
hub or switch is in the same physical broadcast domain. For any device to
transmit data to another host on a different network (broadcast domain), the
devices must be configured with logical network addresses. If not, only
local communication can take place.
Designing broadcast domains is the key to success in any network design and
implementation. Typically, a broadcast domain should be created by groups of
users that share the same network resources. This is not always possible
with routers because network broadcast domains are created by physical
location. Take a look at Figure C. Notice that each floor has its own
switch and that each switch is connected to the router on the first floor.
This is a good design that has worked well for many years.
Figure C |
|
The first floor holds the finance
department, the second floor holds the sales department, and the
third floor holds the marketing department. |
Also in Figure C, notice how each floor holds a different department.
Everything works well since each floor is its own broadcast domain. However,
problems can occur if you have to place users from one department into the
wrong broadcast domain.
What if, for example, a salesperson was hired to work
with the sales team, but the second floor has no room for the new
salesperson? The accounting department, which sits on the first floor, has
plenty of room, and this new salesperson now sits on the first floor. Where
do you plug in the computer for this new salesperson? This salesperson's
computer is placed physically into the accounting department's broadcast
domain. For this salesperson to reach the network resources for the sales
department, his or her information must go through the router. This can
cause latency, which is the measurement of time that a packet takes to get
from a transmitting device to a receiving device. This is not the biggest
problem, however. When the sales server, which sits on the second floor,
broadcasts application information designed to provide network services to
the sales force, the salesperson on the first floor does not see the
broadcast.
To solve this problem, you could make the accounting department's broadcast
domain part of the sales department's broadcast domain by connecting the two
networks together without the router, but that would defeat the purpose of
creating broadcast domains for better network response time and security. Do
you really want all those salespeople on the same network as the
accountants? Another solution may be to run a cable to the second floor
switch so that the new salesperson is plugged in to the right network. A
third solution would be to add another switch on the first floor that plugs
in to a fourth router interface, which would create a new broadcast domain.
The salesperson would have to go through the router to send and receive any
information via computer, but at least the computer would not be part of the
accounting broadcast domain.
All of the solutions I provided above are terrible! You're probably saying,
"There has got to be a better way," and there is. If you have a business
requirement problem of creating broadcast domains by physical location, you
can instead create Virtual LANs (VLANs) with Layer 2 switches. A VLAN is a
broadcast domain created in a Layer 2 switched internetwork.
The beauty of VLANs is that you do not have to create broadcast domains by
physical location like you do with routers. You can create VLANs by
assigning any switch port, on any switch in your internetwork, to any VLAN.
Figure D shows how our network could look if we used VLANs instead of
router interfaces to break up our broadcast domains.
Figure D |
|
This is a look at how a network might look
if we used VLANs to break up our broadcast domains. |
Notice that each floor has a salesperson, marketing person, and accounting
person, and each person is in the appropriate broadcast domain. Although you
can still create broadcasts by physical location with VLANs, the solution I
showed you here is the most typical configuration. This is dependant solely
on the business requirements, however.
Do you still need routers if you are using VLANs? Absolutely! For the
broadcast domains to communicate, a Layer 3 device is needed. There are a
few different ways to configure the router for inter-VLAN communication, and
I will cover this in a future article. Just understand that you need a
router, or Layer 3 card in a switch, for a device on each VLAN to
communicate to another device in a different VLAN.
Routers are packet switches!
Routers are called routers because their main purpose in life is to
route data from one network to another network. To be able to move data,
however, a router must switch packets of information from the interface it
was received on to the destination exit interface. This is called packet
switching.
Obviously, this is different from frame (LAN) switching. Frame switching
uses only the hardware destination address found in the frame header to find
the exit port on the switch. A router uses the logical destination address
found in the packet header to find the exit interface on the router.
Routers create maps of the internetwork
By running routing protocols, the router learns about remote networks
from neighbor routers. The router then builds a routing table that describes
how to find the remote networks. If the network is directly connected, the
router already knows how to get to the network. If the networks are not
attached, the router must learn how to get to the remote network with either
static routing—which means that the administrator must manually type all
network locations into the routing table—or the administrator can turn on
dynamic routing.
Dynamic routing is used to allow routers to broadcast information about all
the networks known by this transmitting router to neighboring routers. The
receiving router (or routers), in turn will add these routes into its own
routing table and then broadcast this information to its neighbors, which
will add the routes to its routing table. Any changes that occur in the
network are automatically propagated to all routers through a routing
protocol like RIP, IGRP, or OSPF, for example. If a router is not running a
routing protocol, but instead, static routing, the administrator has to make
all changes to all routers by hand.
The routers can only send packets to remote networks by looking at the
routing table and finding out how to get to the remote networks. What
happens when a router receives a packet with a network that is not listed in
the routing table? It discards it! It doesn't send a broadcast looking for
the remote network—the router just throws it out. Period.
Conclusion
Routers are an important part of the Internet and any medium- to
large-size network. By understanding the configuration of the different
protocols used with routers, you prepare yourself for a promising and
prosperous career; however, studying and getting as much hands-on experience
is a must before working on large, corporate-routed networks.
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