GNS3 Talks: Python for Network Engineers with GNS3 (Part 4) - Create switch VLANs using loops

Key Takeaways

this is one of multiple videos teaching you network programmability using Python and genus [Music] [Music] 3 now in the previous video we created switch one script 2. py and I told you that it's inefficient to create a vlans in this way on a switch now it works so as an example we can use the command S1 script 2. py and the script SC will tnet to the switch and create various vlans but it's an inefficient way of adding vlans or doing repetitive work let's see if we can clear up this code and do it better now to do that we're going to create a loop a loop is essentially telling the computer to do something for a certain amount of time so in this example I'm going to do a range I'm going to say do the following in the range 2 to 10 now in Python spacing is very important think about how the Cisco iOS works or the iOS of many networking devices work when we looking at code so as an example this code or configuration if you prefer of a CSCO router do you agree that this space is very important this space indicates that this IP address is configured under this loop back all this code here belongs to this interface and so forth and so on we know that this network command belongs under the OSF writing process if I try to type Network 0.0.0.0 here in global configuration mode the router doesn't accept it that space is telling us that this piece of code or configuration belongs under the writing process this code belongs under this interface so use that as an analogy for what we're doing here the code that we're now going to write belongs under the for Loop so what we're going to write to the switch is VLAN space plus n plus carriage return so those two lines will allow us to replace the code that we've got here what I'll do is paste that back but notice I'm going to press Tab and what we want to do here is say plus n Plus carriage return now when we run this script it's going to fail because this is an integer and this is a string and I'll talk about that in a moment but essentially what we've done is we've replaced all of this code with three lines and we can extend that by for instance doing 2 to 20 so what I'll do is I'll save that code and I'll run it but notice the problem when we run the code we told that line 23 in our script in other words this line is mixing strings and integers so I'll run python directly and notice what we told when we enter type one in Brackets we told that this is an integer type David that's a string or type this way is a string but type let's say 1.5 is a floating value integers are whole numbers floating numbers as an example allow us to have fractions in our numbers now we told once again that we can't mix string and integer objects so we're not going to let that stop us what we're going to do here is change the integer to a string and all we're going to do is type string or Str Str and brackets around that n to change it to a string value and save the script now on the switch at the moment show VLAN shows us that we have ethernet vlans 1 to8 configured we're not going to worry about token ring and fddi but what happens when we run our script so David put the password in and notice what happens it's creating vlans 2 to9 so previously we had one to8 now show VLAN brief shows us that we have a VLAN 9 and this is the power of Loops if we want to add let's say vlans 2 to 20 we simply change that one value run our script again notice it's created VLS 2 to 19 so show vline brief notice the VLS are created and one thing I should say is if we want to create two to 20 we need to add one extra value there so let's run the script again this is the great thing about programming if you make a mistake you can just correct it and what's great with gns3 is you can actually see what your program or script is doing so notice we have VLAN 20 created so it was as simple as that to write a script that creates multiple vlans on a cisa switch it's not a very long script it only has a few lines in it let's be a little bit ridiculous now and this is the power of programming I'll create 100 vlans on our switch so run the script again and while that's running in the background notice the VLS are being created we can see 27 VLS now 32 VLS now we at 36 now you may say that you could copy and paste from an Excel spreadsheet but remember with loops we can now create a Loops within Loops so what we could do is rather than just connecting to one switch we can Loop across multiple switches and run this code across multiple switches hope you enjoyed this video if it was of benefit to you please like it and please subscribe to my YouTube channel I wish you all the very best