There’s a belief among Cisco CCIE Lab exam candidates that you have to get all the core routing technologies correct on your exam before you can even consider doing things like IP services, QOS, or security topics. That’s a myth. Sometimes, you’ll get core routing questions in the exam that are extremely important to the functionality of the lab, but you will need to learn how to solve them. I want to tell you two ways to work around those problems. These help you pass the lab. The first is doing the wrong thing on purpose, and the second is not doing the right thing. This sounds counterintuitive, but I want to walk you through two examples. Suppose we get a question like this on the lab exam.
It says– to ensure host one can ping host 2. You can see from this network that we’ve got four routers. They’re running OSPF. We’ve got three different areas. It’s a basic design. We have a limitation that we cannot create any tunnel interfaces. So, we need to solve this using some intrinsic OSPF features. There might be an obvious solution to this problem. Maybe you tried that solution but can’t get it working, or perhaps you don’t know the answer, and you’re stuck early in the CCIE Lab exam. You’re starting to panic because you don’t know how to overcome this problem.
If you’ve been studying for the CCIE Lab for any period, the desired answer is an OSPF virtual link from r2 to r3. This would connect r3 back to the backbone, it would make r3 an abr, and it would allow area 2 and area 0 to communicate across area one. However, like I said before, if you try to configure a virtual link and it doesn’t come up, there could be many reasons. For example, there could be an OSPF path cost that’s greater than 65535 across area 1, there could be some ACL that’s blocking the unicast OSPF messages between r2 and r3, there could be some phantom authentication enabled for area 0 that you didn’t know about, there’s a million things that could go wrong.
If you can’t get it working and you want to move on to other tasks, do the thing that they told you not to do. Create a GRE tunnel from r2 to r3 and enable OSPF area 0. Yes, this is not the same thing as a virtual link, and it has different forwarding characteristics and has a different impact on the OSPF database. I am not saying that they are precisely the same. They are similar enough that you would be able to ping from host 1 to host 2, and you would be able to continue with the CCIE Lab. You will forfeit the points for this question, but it will allow you to move on. Because there are a lot of issues later in the test with the security, the IP services, the QOS, etc. You need to get to that point in the test to pass. This approach allows you to trade off two or three points to gain 20. So it’s worth doing if you need to learn how to continue. The second example is, don’t do the right thing. It’s the opposite of what we just discussed. Rather than a task restricting what you can do, it might tell you precisely what you must do.
For example, in the above topology, it asks you to configure PPPoE from r1 to r2. That tells you exactly what to do. Then, it means you can make some modifications in CCIE Lab, like enabling OSPF on it, with some specific cost. Now, you need to read between the lines here. You have a limited amount of flexibility here. It’s telling you to do a particular thing. But, the relevance of the OSPF cost might be significant later. For instance, all those other links in the topology have various prices, and the lab designer is trying to make it so that your routing is asymmetric across r1 to r2 for some reason. This means that the OSPF cost is significant.
But if you need help getting PPPoE working, how is OSPF routing going to work? In this situation, don’t do it. Skip PPPoE if you don’t know how to configure it or if it doesn’t work. Don’t waste any time trying to make this happen. Just work around it. The trick is to make the interface look as much like a PPPoE connection as possible. That means using an OSPF point-to-point network type, configuring the manual cost of 30 that they wanted, and adjusting the MTU. PPPoE comes with an 8-byte overhead encapsulation. So you want to be sure to include that. By making that link look like a PPPoE connection to the maximum extent possible, the other routers in the topology will be able to function the way they should. This will provide the correct routing in CCIE Lab.
If any data flows across it, maybe later in the lab, they want you to deal with some TCP and MSS adjustments so the MTU becomes relevant. The link looks like it should, even if you need help understanding the core technology. Again, you’ll forfeit the points for this question, so you’ll lose two or three points, but it opens up another 20 moments later in the test that you can at least attempt.
I want to avoid droning on about this. If you’re studying for the CCIE Lab, you’re already busy. You’re already looking all the time, and I don’t want to make that worse by talking needlessly. So, I’m going to end it here. When you’re performing your studies and attempting a full-scale practice lab, try to incorporate these techniques into your studies and see if it gives you a better score or allows you to finish the test with some more time left over. If you can complete most of the exam with time left over, you can always come back to these tasks later and try to fix them and make them work, and then redo your verification. The thing you absolutely must avoid is keeping these from stopping you from being dead in your tracks. Because in that case, you’ll have no chance of passing.