Switches in GNS3

I’ve been getting questions about switches in GNS3 so decided to write this article explaining which switching protocols you can or cannot use in GNS3.

First of all, it’s impossible to run switches in GNS3! That being said the option that we do have is using the NM-16SW switching module in a router which gives us some capabilities. Now you are probably wondering ‘which capabilities’ so let’s have a look what we can do. You might have noticed that in the newer versions of GNS3 there’s a Etherswitch Router. This is a Cisco 3725 with the NM-16SW module so only the icon is different.

Now let’s see what “switching” options we have.

• Vlan interfaces: You can create vlan interfaces which can be used as the default gateway for clients. For example interface vlan 10 with IP address 10.10.10.10 and interface vlan 20 with IP address 20.20.20.20. This is the same as you would do on any L3 switch like the 3550 or 3560.

• Trunking: Trunking is looking good, the only encapsulation type is 802.1Q which is fine (no ISL). But you are able to setup a trunk connection just fine. There is no dynamic auto or dynamic desirable mode so now way to lab this.

• Vlan Trunking Protocol (VTP): VTP is working just like on a switch, you can configure the VTP domain, mode like server / client / transparent, pruning and passwords.

• Etherchannel: If you want to learn etherchannel you are better off with some real switches, you can enable it but there’s no Lacp or Pagp support.

• Spanning-Tree: You can learn the basics of spanning-tree, see which ports are in blocking and forwarding mode but don’t expect any fancy stuff. To be honest you are better of with real switches to study spanning-tree if you ask me.

• Switched Port Analyzer: Should be possible but to be honest I didn’t fully test it yet. The commands are there so I believe it should work just fine.

• Storm Control: Storm control is there and seems to be just fine.

• Quality of Service (QOS): The switch commands for Qos are there like “mls qos” and you can configure the mls qos maps. This looks cool to me!

So what do I think of it? To be honest I like it a bit better after researching all the commands and what the options are. If you want to play with vlans or vlan interfaces there’s nothing wrong using the NM16-SW module. Trunking is fine, you can do everything that you want as long as you don’t care about ISL.

VTP is the same thing as on a real switch so no need for real hardware here. Etherchannel I don’t like since there’s no Lacp or Pagp support, the same for spanning-tree…you lack a lot of commands and there’s no way to switch to other modes like MST which you can do on a real switch. SPAN is excellent and I also like Storm Control and the Quality of Service commands.

So what should you do? get real switches or use GNS3? If you have no budget and are studying for CCNA, just use GNS3. If you do have some money to spend get 2 or 3 Cisco 2950’s and you will have more fun. If you are studying for CCNP SWITCH just get yourself a 3550 and a 2950 or something. It’s not too expensive and using a real switch is better.

For the CCIE…do yourself a favor and invest in 2x 3560 and 2x 3550 and connect them to a breakout switch or get quad-nics. You’ll be spending a LOT of time studying and it’s better to spend your time at switches than the NM16-SW module.

If you want to know all the options for the NM16-SW module check out this article on the Cisco website: http://www.cisco.com/en/US/docs/ios/12_2t/12_2t8/feature/guide/ft1636nm.html#wp1433377

Next time i’ll be writing an article how to connect your real physical switches to your GNS3 topology.

So what do you think? Is there anything i’m missing? Please share your opinion in the comments!

Rene

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Written by René Molenaar - CCIE #41726

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