Internetworking Basics
Written on 15:19 by Ahmet Emir
you’ve got to accept the big account and apprentice the acknowledgment to the key question, Why is it
so important to apprentice Cisco internetworking?
Networks and networking accept developed exponentially over the aftermost 15 years—understandably
so. They’ve had to advance at ablaze acceleration aloof to accumulate up with huge increases in basal missioncritical
user needs such as administration abstracts and printers as able-bodied as added avant-garde demands such as
videoconferencing. Unless anybody who needs to allotment arrangement assets is amid in the same
office breadth (an added aberrant situation), the claiming is to affix the sometimes
many accordant networks calm so all users can allotment the networks’ wealth.
Starting with a attending at Amount 1.1, you get a account of a basal LAN arrangement that’s connected
together application a hub. This arrangement is absolutely one blow area and one broadcast
domain—but no worries if you accept no abstraction what this agency because I’m activity to allocution about
both blow and advertisement domains so abundant throughout this able chapter, you’ll probably
even dream about them!
FIGURE 1 . 1
The basal network
Okay, about Amount 1.1… How would you say the PC alleged Bob communicates with the
PC alleged Sally? Well, they’re both on the aforementioned LAN affiliated with a multiport captive (a
hub). So does Bob aloof accelerate out a abstracts message, “Hey Sally, you there?” or does Bob use Sally’s
IP abode and put things added like, “Hey 192.168.0.3, are you there?” Hopefully, you picked
the IP abode option, but alike if you did, the account is still bad—both answers are wrong!
Why? Because Bob is absolutely activity to use Sally’s MAC abode (known as a hardware
address), which is austere appropriate into the arrangement agenda of Sally’s PC, to get ahold of her.
Great, but how does Bob get Sally’s MAC abode aback Bob knows alone Sally’s name and
doesn’t alike accept her IP abode yet? Bob is activity to alpha with name resolution (hostname to
The basal arrangement allows accessories to allotment information.
The appellation computer accent refers to bifold cipher (0s or 1s).
The two hosts aloft acquaint application accouterments or MAC addresses.
(Hub)
Bob Sally
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5
IP abode resolution), article that’s usually able application Area Name Service
(DNS). And of note, if these two are on the aforementioned LAN, Bob can aloof advertisement to Sally asking
her for the advice (no DNS needed)—welcome to Microsoft Windows (Vista included)!
Here’s an achievement from a arrangement analyzer depicting a simple name resolution action from
Bob to Sally:
Time Antecedent Destination Agreement Info
53.892794 192.168.0.2 192.168.0.255 NBNS Name concern NB
SALLY
<00>
As I already mentioned, aback the two hosts are on a bounded LAN, Windows (Bob) will just
broadcast to boldness the name Sally (the destination 192.168.0.255 is a advertisement address).
Let’s booty a attending at the blow of the information:
EthernetII,Src:192.168.0.2(00:14:22:be:18:3b),Dst:Broadcast (ff:ff:ff:ff:ff:ff)
What this achievement shows is that Bob knows his own MAC abode and antecedent IP abode but
not Sally’s IP abode or MAC address, so Bob sends a advertisement abode of all
f
s for the MAC
address (a Abstracts Link band broadcast) and an IP LAN advertisement of 192.168.0.255. Again,
don’t freak—you’re activity to apprentice all about broadcasts in Affiliate 3, “Subnetting, Variable
Length Subnet Masks (VLSMs), and Troubleshooting TCP/IP.”
Before the name is resolved, the aboriginal affair Bob has to do is advertisement on the LAN to get
Sally’s MAC abode so he can acquaint to her PC and boldness her name to an IP address:
Time Antecedent Destination Agreement Info
5.153054 192.168.0.2 Advertisement ARP Who has 192.168.0.3? Acquaint 192.168.0.2
Next, analysis out Sally’s response:
Time Antecedent Destination Agreement Info
5.153403 192.168.0.3 192.168.0.2 ARP 192.168.0.3 is at 00:0b:db:99:d3:5e
5.53.89317 192.168.0.3 192.168.0.2 NBNS Name concern acknowledgment NB
192.168.0.3
Okay sweet— Bob now has both Sally’s IP abode and her MAC address! These are both
listed as the antecedent abode at this point because this advice was beatific from Sally aback to
Bob. So,
finally
, Bob has all the appurtenances he needs to acquaint with Sally. And aloof so you
know, I’m activity to acquaint you all about ARP and appearance you absolutely how Sally’s IP abode was
resolved to a MAC abode a little afterwards in Affiliate 6, “IP Routing.”
By the way, I appetite you to accept that Sally still had to go through the aforementioned resolution
processes to acquaint aback to Bob—sounds crazy, huh? Consider this a acceptable to IPv4
and basal networking with Windows (and we haven’t alike added a router yet!).
To complicate things further, it’s additionally acceptable that at some point you’ll accept to breach up one large
network into a agglomeration of abate ones because user acknowledgment will accept dwindled to a apathetic clamber as
the arrangement grew and grew. And with all that growth, your LAN’s cartage bottleneck has reached
epic proportions. The acknowledgment to this is breaking up a absolutely big arrangement into a cardinal of smaller
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Internetworking
ones—something called
network segmentation
. You do this by application accessories like
routers
,
switches
,
and
bridges
. Amount 1.2 displays a arrangement that’s been anecdotal with a about-face so anniversary network
segment affiliated to the about-face is now a abstracted blow domain. But accomplish agenda of the actuality that
this arrangement is still one advertisement domain.
FIGURE 1 . 2
A about-face can alter the hub, breaking up blow domains.
Keep in apperception that the hub acclimated in Amount 1.2 aloof continued the one blow area from the
switch port. Here’s a account of some of the things that frequently account LAN cartage congestion:
Too abounding hosts in a advertisement domain
Broadcast storms
Multicasting
Low bandwidth
Adding hubs for connectivity to the network
A agglomeration of ARP or IPX cartage (IPX is a Novell agreement that is like IP, but really, really
chatty. About not acclimated in today’s networks.)
Take addition attending at Amount 1.2—did you apprehension that I replaced the capital hub from Amount 1.1
with a switch? Whether you did or didn’t, the acumen I did that is because hubs don’t articulation a network;
they aloof affix arrangement segments together. So basically, it’s an bargain way to connect
a brace of PCs together, which is abundant for home use and troubleshooting, but that’s about it!
Now routers are acclimated to affix networks calm and avenue packets of abstracts from one
network to another. Cisco became the de facto accepted of routers because of its high-quality
router products, abundant selection, and absurd service. Routers, by default, breach up a
broadcast
domain
—the set of all accessories on a arrangement articulation that apprehend all the broadcasts beatific on
that segment. Amount 1.3 shows a router in our little arrangement that creates an internetwork and
breaks up advertisement domains.
(Hub)
Switch
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Chapter 1
Internetworking
frames, routers (layer 3 switches) use analytic acclamation and accommodate what is alleged packet
switching. Routers can additionally accommodate packet clarification by application admission lists, and aback routers
connect two or added networks calm and use analytic acclamation (IP or IPv6), this is called
an internetwork. Last, routers use a acquisition table (map of the internetwork) to accomplish path
selections and to advanced packets to alien networks.
Conversely, switches aren’t acclimated to actualize internetworks (they do not breach up broadcast
domains by default); they’re active to add functionality to a arrangement LAN. The capital purpose
of a about-face is to accomplish a LAN assignment better—to optimize its performance—providing more
bandwidth for the LAN’s users. And switches don’t advanced packets to added networks as
routers do. Instead, they alone “switch” frames from one anchorage to addition aural the switched
network. Okay, you may be thinking, “Wait a minute, what are frames and packets?” I’ll tell
you all about them afterwards in this chapter, I promise!
By default, switches breach up
collision domains
. This is an Ethernet appellation acclimated to describe
a arrangement book wherein one accurate accessory sends a packet on a arrangement segment, forcing
every added accessory on that aforementioned articulation to pay absorption to it. At the aforementioned time, a different
device tries to transmit, arch to a collision, afterwards which both accessories charge retransmit, one
at a time. Not actual efficient! This bearings is about begin in a hub ambiance area each
host articulation connects to a hub that represents alone one blow area and alone one broadcast
domain. By contrast, anniversary and every anchorage on a about-face represents its own blow domain.
Switches actualize abstracted blow domains but a distinct advertisement domain.
Routers accommodate a abstracted advertisement area for anniversary interface.
The term
bridging
was alien afore routers and hubs were implemented, so it’s pretty
common to apprehend bodies apropos to bridges as switches. That’s because bridges and switches
basically do the aforementioned thing—break up blow domains on a LAN (in reality, you cannot buy
a concrete arch these days, alone LAN switches, but they use bridging technologies, so Cisco
still calls them multiport bridges).
So what this agency is that a about-face is basically aloof a multiple-port arch with added brainpower,
right? Well, appealing much, but there are differences. Switches do accommodate this function,
but they do so with abundantly added administration adeptness and features. Plus, best of the time,
bridges alone had 2 or 4 ports. Yes, you could get your easily on a arch with up to 16 ports,
but that’s annihilation compared to the hundreds accessible on some switches!
You would use a arch in a arrangement to abate collisions aural broadcast
domains and to access the cardinal of blow domains in your network.
Doing this provides added bandwidth for users. And accumulate in apperception that using
hubs in your arrangement can accord to bottleneck on your Ethernet network.
As always, plan your arrangement architecture carefully!
Figure 1.4 shows how a arrangement would attending with all these internetwork accessories in place.
Remember that the router will not alone breach up advertisement domains for every LAN interface,
it will breach up blow domains as well.
Internetworking Basics
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FIGURE 1 . 4
Internetworking devices
When you looked at Amount 1.4, did you apprehension that the router is begin at centermost date and
that it connects anniversary concrete arrangement together? We accept to use this blueprint because of the
older technologies involved–—bridges and hubs.
On the top internetwork in Amount 1.4, you’ll apprehension that a arch was acclimated to affix the
hubs to a router. The arch breach up blow domains, but all the hosts affiliated to both
hubs are still awash into the aforementioned advertisement domain. Also, the arch alone created two
collision domains, so anniversary accessory affiliated to a hub is in the aforementioned blow area as every
other accessory affiliated to that aforementioned hub. This is absolutely appealing lame, but it’s still bigger than
having one blow area for all hosts.
Notice article else: The three hubs at the basal that are affiliated additionally affix to the
router, creating one blow area and one advertisement domain. This makes the bridged network
look abundant bigger indeed!
Although bridges/switches are acclimated to articulation networks, they will not isolate
broadcast or multicast packets.
Router
Bridge
Switch
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FIGURE 1 . 3
Routers actualize an internetwork.
The arrangement in Amount 1.3 is a appealing air-conditioned network. Anniversary host is affiliated to its own collision
domain, and the router has created two advertisement domains. And don’t balloon that the router provides
connections to WAN casework as well! The router uses article alleged a consecutive interface
for WAN connections, specifically, a V.35 concrete interface on a Cisco router.
Breaking up a advertisement area is important because aback a host or server sends a network
broadcast, every accessory on the arrangement charge apprehend and action that broadcast—unless you’ve got
a router. Aback the router’s interface receives this broadcast, it can acknowledge by basically saying,
“Thanks, but no thanks,” and abandon the advertisement after forwarding it on to added networks.
Even admitting routers are accepted for breaking up advertisement domains by default, it’s
important to bethink that they breach up blow domains as well.
There are two advantages of application routers in your network:
They don’t advanced broadcasts by default.
They can clarify the arrangement based on band 3 (Network layer) advice (e.g., IP address).
Four router functions in your arrangement can be listed as follows:
Packet switching
Packet filtering
Internetwork communication
Path selection
Remember that routers are absolutely switches; they’re absolutely what we alarm band 3 switches
(we’ll allocution about layers afterwards in this chapter). Unlike band 2 switches, which advanced or filter
A router creates an internetwork and
provides access to WAN services.
Switch
Switch
Serial 0
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Internetworking
The best arrangement affiliated to the router is the LAN about-face arrangement on the left. Why?
Because anniversary anchorage on that about-face breach up blow domains. But it’s not all good—all devices
are still in the aforementioned advertisement domain. Do you bethink why this can be a absolutely bad thing?
Because all accessories charge accept to all broadcasts transmitted, that’s why. And if your broadcast
domains are too large, the users accept beneath bandwidth and are appropriate to action added broadcasts,
and arrangement acknowledgment time will apathetic to a akin that could account appointment riots.
Once we accept alone switches in our network, things change a lot! Amount 1.5 shows the network
that is about begin today.
FIGURE 1 . 5
Switched networks creating an internetwork
Okay, actuality I’ve placed the LAN switches at the centermost of the arrangement apple so the routers
are abutting alone analytic networks together. If I implemented this affectionate of setup, I’ve created
virtual LANs (VLANs), article I’m activity to acquaint you about in Affiliate 9, “Virtual LANs
(VLANs).” So don’t stress. But it is absolutely important to accept that alike admitting you have
a switched network, you still charge a router to accommodate your inter-VLAN communication, or
internetworking. Don’t balloon that!
Obviously, the best arrangement is one that’s accurately configured to accommodated the business requirements
of the aggregation it serves. LAN switches with routers, accurately placed in the network,
are the best arrangement design. This book will advice you accept the basics of routers and
switches so you can accomplish tight, abreast decisions on a case-by-case basis.
Let’s go aback to Amount 1.4 again. Looking at the figure, how abounding blow domains and
broadcast domains are in this internetwork? Hopefully, you answered nine blow domains
and three advertisement domains! The advertisement domains are absolutely the easiest to see because
only routers breach up advertisement domains by default. And aback there are three connections,
that gives you three advertisement domains. But do you see the nine blow domains? Aloof in case
that’s a no, I’ll explain. The all-hub arrangement is one blow domain; the arch network
equals three blow domains. Add in the about-face arrangement of bristles blow domains—one for
each about-face port—and you’ve got a absolute of nine.
Router
Internetworking Models
11
Now, in Amount 1.5, anniversary anchorage on the about-face is a abstracted blow area and anniversary VLAN
is a abstracted advertisement domain. But you still charge a router for acquisition amid VLANs. How
many blow domains do you see here? I’m counting 10—remember that connections
between the switches are advised a blow domain!
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