CCIE lab shortcuts - tclsh

Well, being as we all know lab time is precious we're all looking for CCIE shortcuts. Certainly in my labs (passed third time) I found the last hour was usually spent pinging everything to make sure I had that special full routing table. So, you've seen those practice labs right? You've probably gotten hold of a training package from one of the big Cisco training vendors like InternetworkExpert or IPExpert right? Well those full labs they throw at you are pretty extensive and contain a lot of IP addresses. If we were to assume that the real lab has just as many IP addresses then how could you possibly remember all of those. Not only that but could you really be up to the task of pinging each and everyone from each and every layer 3 device on the network to test for full reachability?

Well of course not, that would be nuts. So here is what I did for my lab (frankly I only had enough time to exploit this in my third attempt) and I hope it can help you out. Of course you could always use this on your production network as part of some alarm script or reachability script but I digress.

So, tclsh, what is it and how do we use it? Well it's a very powerful application in it's own right, read about it here. On IOS it runs under it's own binary and is called from privileged EXEC mode. The power of tclsh is not in the scope of this document and honestly I've not even scratched the surface myself. We're going to concentrate on running the application, writing a basic script, executing the script and using it to debug issues in your lab before the time runs out.

So first off how do we call tclsh? From privileged EXEC mode type 'tclsh' then press return



You see you get dropped into a sub-prompt or actually the TCLSH command shell. It is inside here that you can create your script. For our example here I've built up a small three node network. R1, R2 and R3 are connected together using subnets 10.1.12.0/24 (R1<->R2) and 10.1.23.0/24 (R2<->R3) and we are running EIGRP across all three. Each has a local loopback0 interface. Lo0 on R1 is 1.1.1.1, Lo0 on R2 is 2.2.2.2 and Lo0 on R3 is 3.3.3.3.

Here is a diagram so we can see the topology:



eigrp process ID 100 is running on each router. Here is the routing table on R1:



Ok, so in the lab you're going to be busy, too busy to remember IP addresses and *way* to busy to type these all in. Here is a tip. As you go through the lab have notepad open and type in the IP addresses for all of your nodes. I'd use notepad a lot if I were you. There was a study done (can't recall the publisher sorry) which stated that more than 60% of exam (not just CCIE) error was down to mistyping or human errors (the technical term for this of course is "fat fingers"). So why would you set yourself up for this sort of thing - put it in notepad and minimise your risk.

Here is Fred who could press QWASEDZXC with one finger easily.



So anyway you've got a list of IP addresses - this is perfect. At the end of your lab you're going to be ahead of the game when it comes to troubleshooting with tclsh!

The basic setup here for our tclsh script is we are going to run an iterated 'for' loop. For any programmers out there you'll be aware of 'for' loops. The idea behind a loop is to reduce the number of lines of code for repetitious process. e.g. "check A looks like B" or "doesn't look like B", or maybe "is smaller than B", or "bigger than B" etc. the point is that whatever the differentiator is between A and B do it, and do it again until the end...whatever the end is. In our ping script we're not doing any comparison we're just being iterative through a list of address. Hmm this is getting hard to put down in words I guess so here is our example:

We've got a number of IP addresses in our topology and these are in our notepad list:

1.1.1.1
2.2.2.2
3.3.3.3
10.1.12.1
10.1.12.2
10.1.23.2
10.1.23.3

Cool. Right now the syntax for our for loop starts with a 'foreach VARIABLE_NAME'. Now the 'VARIABLE_NAME' can be anything but it's usual to pick something small like a letter e.g. a or b, most guides I've seen us 'i', but it could easily be 'ipaddress' or even 'ccie'...you choose. The VARIABLE_NAME is just something you are associating with your IP addresses i.e. the first IP address in your list, for the first run through the loop will associate with the letter 'a' or in other words a=1.1.1.1.

The tclsh script ends by telling IOS what command it is supposed to run against the variable in the script. Our script is written to 'ping' so clearly we'll need to have ping in there somewhere ;-)

Here is the command based on us choosing a VARIABLE_NAME of 'a'. If you chose something else here then you should substitute it.

"ping $a repeat 1"

Lets walk through this. The tclsh parser will send the string "ping $a repeat 1" into the privileged EXEC mode just as if you were typing it. Now remember $a takes on the IP address for each iteration through the loop so first round $a=1.1.1.1 so now the line makes more sense "ping 1.1.1.1 repeat 1". We are saying repeat 1 because we don't need IOS to do the usual 5 pings...we're running low on time right?! So we rung one ping and we look for a "!" which means all good. Now some people tell me "Hey, sometimes the first ping is lost because the router needs to lookup the MAC address and it times out". Well I say use whatever works best for you. In the lab it's likely your routers will already have the MAC addresses they needs due to routing protocols and other troubleshooting you already did so one ping should be enough.

Right, we've been through the logic behind the script. Here is is being used.



Thats it - all pinged - I guess I passed ;-)

The tclsh application is way more powerful than this but for an exam tool to save you some time this is probably as hard as you need to go into it. For a more in depth study of tclsh in IOS and how powerful it can be for operation use then I can recommend this Cisco Press book.