gns3 - creating the simplest topology

7/25/2019 GNS3 - Creating the Simplest Topology

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Creating the Simplest Topology

1. Introductionon page 1 2. Configuring a routeron page 2 3. Apply an idlepc valueon page 9

4. Creating a simple connection in the topologyon page 12 5. Configuring the routerson page 16

1. Introduction

We describe how to build a more complex topology later, but for now, lets just learn how to:

Place one router on the desktop,

Start it, and

Console into it.

When an IOS is running, it will consume up to 100% of your CPU time. This will cause your computer to

become very sluggish and will prevent building more complex topologies. However, if we use an idlepc value,

we can reduce CPU usage dramatically. It puts the IOS into a sleep state when it is not in active use and

wakes it up only when it is necessary.

A more technical explanation will be given later. The GNS3 window is divided into four panes by default. The

left-most pane lists the types of nodes available. You will see router icons for the various platforms:

PIX and ASA firewalls Ethernet switch ATM Bridge ATM switch Frame Relay switch Cloud, Qemu VirtualBox guests

The right-most pane will provide a topology summary that will be better understood when we built more

complex topologies. For now, just know that the pane exists. The middle section contains two panes. The top

pane is your work area where a topology may be graphically built. The bottom pane, called the Console, showsDynagen at work. Dynagen, as you recall, is the text-based front end to Dynamips. It is the core emulator being

used.


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2. Configuring a router

Click on a router icon under Nodes Types corresponding to the IOS platform you are using. In our example,

we are using a 3725 platform. Drag an appropriate router node type over to the workplace pane in the middle

and let go. We now have a router ready to configure. First Browse for your router Left click on the Router Icon

A panel will slide out from the left side.


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Move your mouse over the new Router labeled c3725. Hold down the left mouse button and drag and drop it

into the center panel. The center panel is your topology panel. The router should be in the topology panel.


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Right-click the router and choose "Configure". Click on R1 and then the Slots tab. Slot0 already has module.

GT96100-FE. A little comment about some of the modules. With the 3725, the first module comes with the

device as a default. If you load any other router image, it will still work, but just know you will have to set the

appropriate module as per the hardware guidelines of a real Cisco device. Some users in the past few months

will start adding modules whether or not it's supported. Dynagen will get error messages coming from the IOS

image. So be careful not to add modules that are not supported, or need more memory.
https://community.gns3.com/servlet/JiveServlet/showImage/102-2129-18-50399/configure.jpg


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Next, click the drop-down arrow next to slot1 and choose NM-4T and NM- 16ESW in slot 2. The NM-4T installs

SERIAL connections and the NM-16ESW sets up a 16 port Ethernet switch module.

Click OK

Some users have asked what is the big deal about adding the modules? Well the reason for adding the

modules is they offer the ability to connect using Serial or Switching. For those not familiar with Cisco modules,

the NM-4T provides 4 Serial connections. In most of the topologies created, since the module is installed in the

Slot '1', all the Serial connections will start with 1/x. Where 1 indicates the slot it's located in, and 0-3 are the

actual ports. So in this case, if you chose this option, your router now has four Serial connections. S1/0, S1/1,

S1/2, S1/3.

If you look closely, you can see another module, the NM-16ESW. This module adds 16 ethernet ports for

Layer 2 switching. You can perform VLAN, Etherchannel and basic switching configurations. This module is

recommended despite the protestations of many experienced S + R Engineers. The reason for including this

is because many students starting off needs hands on experience with configuring vlans and etherchannel as

well as learning to connect end devices. The switch module will forward packets to the router and the router

can perform the basic functions of choosing the correct route. Layer 3 is possible, but requires more advanced


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devices called IOUs which are supported in Linux. You may also look for other integrated virtual machines that

will work like Cisco such as Juniper, Arista, Extreme. These integrated devices are designed to use virtualbox

and you can drop those into your topology and connect them. For now, in the getting started section, have a

basic, all around router with the capability to switch is ideal. As you can see in the previous screenshot, I use

the 3725 for the option to create a layer 2 switch. I have a 7200 IOS image that I can use for a Layer 3 switch.

On the Topology panel, right-click the router and choose Start. The router starts up and goes into a boot

process. Right-click the router again and choose Console. A Putty console opens up. You may need to press

enter once initially in the console window. After a few seconds, your virtual router should have started.


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The next screenshot consoles you into the device as if you were physically next to the router. The engineer that

sets up a router may use the console cable and physically connect to the IOS.
https://community.gns3.com/servlet/JiveServlet/showImage/102-2129-18-50401/r1+start.jpg


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https://community.gns3.com/servlet/JiveServlet/showImage/102-2129-18-50402/r1+console.jpg


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3. Apply an idlepc value

Now its time to choose an Idle-PC value. The Idle-PC command performs analysis on a running IOS to

determine the most likely points in the code that represent an idle loop in the IOS process. Once applied,Dynamips "puts to sleep" the virtual router occasionally when this idle loop is executed. This significantly

reduces CPU consumption on the host without diminishing the virtual routers capacity to perform real work. Now

be sure you can see the prompt of your router in the Putty window.

Finding the right Idle-PC value is a trial and error process, consisting of applying different values and monitoring

the CPU usage. Best Idle-PC values are usually obtained when IOS is in idle state, the following message being

displayed in the console: Hostname con0 is now available

Press RETURN to get started.

Right-click R1 and choose Idle PC.

GNS3 will spend a moment calculating an Idle-PC value before presenting the screen to the right. If you click

the drop-down arrow, you see a list of possible Idle-PC values. Potentially, better Idle-PC values are the ones

with the highest values between the brackets. Choose one of the values with an asterisk (in our example, we

will choose number 1) and click OK.


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To observe CPU usage in Windows;

Press Ctrl+ALT+DEL and choose Task Manager. You can also press: Ctrl+SHIFT+ESC and

get to the Task Manager as well.

Click on the Performance tab to view CPU usage.
https://community.gns3.com/servlet/JiveServlet/showImage/102-2129-18-50403/r1+idle.jpg


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NOTE: In Ubuntu (works on all Linux) use thetopcommand in a console. With OS X, use top -o cpu, or open

the Activity Manager.

You will observe that without an idlepc value, CPU usage will be at or near 100%, but with an idlepc value,

CPU usage will drop to a very low value. Enjoy using your router! You may now return to your Putty window to

use your router. You are actually running the Cisco IOS that you chose.

Please note that Idle-PC values are particular to an IOS image. They will be different for different IOS versions,

and even for different feature-sets of the same IOS version. However, Idle-PC values are not particular to your

host PC, operating system, or to the version of Dynamips that GNS3 is using. It is possible that Dynamips will

not be able to find an Idle-PC value for an image, or that the values it does find do not work. If this happens, try

repeating the process again.

All commands supported by the IOS are available.

Remember earlier we have a default fast ethernet adapter and a four-port serial adapter. If you issue the show

ip interface brief command as shown, youll see the designations for these ports on the router. In our example,they are fa0/0, s1/0, s1/1, s1/2, and s1/3 (in abbreviated form). Pull up the console window you launched

previously and do a show ip interface brief (or sho ip int brief) for short.

Dragging the first router to the topology window and taking a peak at the installed modules is a must before

getting in too deep with designing your topology and finding out a module or something is wrong with launching

the device. Do this test before trying out any new IOS images.

Ok that fine, but a single device is not so useful to create a virtual network! Lets build a more complex topology.

From this point, you are now ready to look at other "cookbooks" and forum discussions tailored to specific

topologies and routing protocols. Happy Hunting!


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4. Creating a simple connection in the topology

Lets build the topology shown with three routers.

Step I:Start GNS3 Step II:Drag three routers (here we are using Router c3725) running an IOS you have configured

into the workspace. Step III:Select the 3 routers, right-click and choose Configure. Step IV:Choose a NM-4T serial adapter for slot1. This should have configured a NM-4Tnetwork

module in slot1 for all the routers. Step V:Now were ready to connect the routers together. Click the Add a link button on the toolbar.

When you click on this, your pointer has cross hairs and stays in this mode until you click the Add a link againto stop this action. For now, you can select the serial connection on one router and drag a line to the next

router.


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https://community.gns3.com/servlet/JiveServlet/showImage/102-2129-18-50406/connect+serial+1.jpg


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Continue connecting one serial connection to each router. The nice thing about simulating networks in GNS3,

is hardware is unlimited. So I use the hardware to keep track of the interfaces that are connected to each other.

For example, S1/0 will be used for the interface relationship between R1 and R2. On the next segment, I may

choose S1/1 on R2 and S1/1 on R3. Finally, for R3, I will use S1/2 with R1's S1/2. See the relationship? It

makes configuring easier knowing which interface goes where without having to look at the interfaces.
https://community.gns3.com/servlet/JiveServlet/showImage/102-2129-18-50407/connect+serial+1+on+router+2.jpg


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Of course, if you are following along with an instructor like from CBTNuggets, Chris Bryant or some other

course, you will need to look at the downloaded config files, analyze the diagram and substitute the interface

for R1 in the example with S1/0. Use GNS3 for note taking by expressly labeling your IP interfaces, and other

information to keep your connections logical. Therefore, GNS3 do not have physical connections, these are

graphical LOGICAL connections to each router emulated via a pipe that runs on your host system. Because we

are not physically next to these devices, it can get confusing real quick. If you ever get lost in your connections,GNS3 has some other ways to verify connections. If you click on GNS3 and move your mouse over the device,

it will actually show a status window showing the connections.

In this screenshot, I went ahead and started up the routers which shows by a green dot at the interfaces. I then

hovered over R3 and I can see the following logical connections:

1. Serial 1/0 is empty.2. Serial 1/1 is connected to R2

3. Serial 1/2 is connected to R14. Serial 1/3 is empty.
https://community.gns3.com/servlet/JiveServlet/showImage/102-2129-18-50408/r3+status.jpg


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As you can see, it's easy to check your connections before you spend the next 20 minutes configuring the IP

address, frame relay settings and other interface related configurations.

Choose Manual from the drop-down menu. Your cursor will change to a cross, and the icon

will change to a red stop sign with an X.

Click on R1 and select s0/0. Slide your mouse over to R2, and then click on R2 and select s0/0.

Click on R2 again and select s0/1. Slide your mouse over to R3,

Click on R3 and select s0/0.

Now click the Add a link button again. (It looks like a red Stop sign with an X. After

clicking on it, the icon will change back to a connector look.

5. Configuring the routers

You may now configure each router. In our example, we will assign IP addresses in the 192.168.1.0/24

network.

Enable RIP (a routing protocol) and finally test connectivity by using pings. It is always good practice to write

out all the IP addresses and subnets BEFOREconfiguring. Many engineers take most of their time in the

planning phase than the actual configuration. Why? Because you will soon find out in many CISCO based

courses, there's like 50 pages explaining the history of the technology, but in reality, you only need to commit

to memory a couple of well placed commands. Examine the screenshot below. I have used GNS3 as a huge

piece of notepaper and typed in notes of value to me.

R1>en

R1#conf t

R1(config)#int s1/0

R1(config-if)#ip add 192.168.1.1 255.255.255.0

R1(config-if)#no sh

R1(config-if)#int s1/2

R1(config-if)#ip add 192.168.3.2 255.255.255.0

R1(config-if)#no shut

R1(config-if)#exit

R1(config)#router rip

R1(config-router)no auto-summaryR1(config-router)version 2

R1(config-router)network 192.168.1.0

R1(config-router)network 192.168.3.0

R1(config-router)exit

R1(config)#exit


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R2>en

R2#conf t

R2(config)#int s1/0

R2(config-if)#ip add 192.168.1.2 255.255.255.0

R2(config-if)#no sh

R2(config-if)#int s1/1R2(config-if)#ip add 192.168.2.1 255.255.255.0

R2(config-if)#no sh

R2(config-if)#exit


R2(config-router)no auto-summary

R2(config-router)version 2

R2(config-router)#net 192.168.1.0


R2(config-router)#exit

R2(config t)exit

R3>en

R3#conf t

R3(config)#int s1/1

R3(config-if)#ip add 192.168.2.2 255.255.255.0

R3(config-if)#no sh

R3(config-if)#int s1/2

R3(config-if)#ip add 192.168.3.1 255.255.255.0

R3(config-if)#no sh

R3(config-if)#exit


R2(config-router)no auto-summary

R2(config-router)version 2



R3(config-router)#exit

R3(config t)exit

Verify these connections by pinging each interface. Then crack open your CCNA or CCENT book and start

playing with this topology. Break it and then fix it. Add a router. Change a routing protocol. You get the idea.

The world is your oyster and your knowledge are the pearls in your adventures!


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Getting started guide

gns3 - creating the simplest topology

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