Simulating a Host in Dynamips/GNS3

For those who don’t know what Dynamips is …. aaaaaaaahhh … it is a very cool piece of software that allows us to emulate any Cisco router and even the PIX. It is a very useful tool for experimenting with Cisco-based networks and one of the cheapest ways to prepare for the CCIE lab. I use it for the later purpose and also to get some hands-on practice with various networking technologies. GNS3 is a very cute graphical front-end to Dynamips which is more suited for adhoc experimentation due to the simply way you can create any topology by dropping devices and connecting them. I was watching a video class on IPv6 when something the instructor said hit me “… without the ipv6 unicast-routing command, this router will not function as a router”! …. immediately I fired up my copy of GNS3 and to simulate a host, I added a router and proceded thus to make it a ‘host’ 1. Disabled routing [no ip routing in global config mode] 2. Configured and IP address and subnet mask on an interface and enabled the interface. 3. Gave the router a default gateway by typing ip default network a.b.c.d where a.b.c.d is the IP address of the router to which this ‘host’ is connected. An viola! I had a host I could ping from. This enables me to simulate a LAN connected to the router … definitely a more elegant way that using loopback addresses which is what I was using until now. So experiment away and drop me any suggestions.

9 thoughts on “Simulating a Host in Dynamips/GNS3

  1. Hello,

    You know alot about Dynamips/GNS3. Have you ever had an issue where you were using a pre-built net file used in Dynamips, and tried to import it using GNS3? doesn’t seem to work for some reason.


    • Hi Vin,

      If the .net file was pre-built on the same PC, there shouldn’t be any problem. If it wasn’t, I’d check my paths to dynamips and images.

      • Hi,

        What I find is that Dynamips .net files and GNS3 .ntes files are not interchangeable. Looks like GNS3 expects different info in the net file itself, that’s what I was getting at.

        For example,
        I tried reading this in, (had to change the path to the images)
        # Simple Ethernet Switch lab
        autostart = false
        ghostios = true


        image = /opt/dynagen/images/c7200-p-mz.120-32.S5.image
        ram = 64
        npe = npe-400
        disk0 = 0
        disk1 = 0
        # idlepc = 0x6065cea4
        idlepc = 0x6058b478

        [[ROUTER PE1]]
        F0/0 = S1 1
        A1/0 = BB2-CE3 A1/0
        S2/0 = ASBR1 S2/2

        [[ROUTER PE2]]
        #image = \Program Files\Dynamips\images\c7200-js-mz.123-21.image
        #ram = 128
        #idlepc = 0x60bb54c0
        F0/0 = S1 2
        F1/0 = BB1-CE5 F0/0

        [[ROUTER PE3]]
        F0/0 = S1 3

        [[ROUTER RR]]
        F0/0 = S1 4

        [[ROUTER BB2-CE3]]

        [[ROUTER ASBR1]]
        S2/0 = ASBR2 S2/0
        S2/1 = ASBR2 S2/1

        [[ROUTER ASBR2]]
        F0/0 = S1 5

        [[ROUTER PE4]]
        F0/0 = S1 6

        [[ROUTER CE6]]
        F0/0 = S1 7

        [[ROUTER BB3-CE7]]
        F0/0 = S1 8

        [[ROUTER BB1-CE5]]

        [[ROUTER CE8]]
        #image = \Program Files\Dynamips\images\c7200-js-mz.123-21.image
        #ram = 128
        #idlepc = 0x60bb54c0
        F0/0 = S1 9
        S1/0 = CE2 S1/0

        [[ROUTER CE2]]
        F0/0 = S1 10

        [[ROUTER CE1]]
        F0/0 = S1 11

        [[ethsw S1]]
        1 = dot1q 1 # PE1 F0/0
        2 = dot1q 1 # PE2 F0/0
        3 = dot1q 1 # PE3 F0/0
        4 = dot1q 1 # RR F0/0
        5 = access 240 # ASBR2 F0/0
        6 = dot1q 1 # PE4 F0/0
        7 = dot1q 1 # CE6 F0/0
        8 = access 300 # BB3-CE7 F0/0
        9 = access 82 # CE8 F0/0
        10 = access 23 # CE2 F0/0
        11 = access 13 # CE1 F0/0

        And I get an exception error in GNS3, but I think it works just fine in Dynamips/Dynagen.



  2. i followed the instruction but, but the message i got was that ‘ip address may not be added to an L2 link’ i was using a fastethernet port.

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