A few days ago I did encounter a task in one of these preparation workbooks for CCIE R&S Lab which was very tricky. This task is not a hard one and does not require extensive knowledge (it can be a CCNP level question as well), but the way the question is formulated can create confusion.I would appreciate your comments on this topic.
The task states that I have a number of subnets from which the followings are important for
10.4.1.4/30 10.4.2.8/29 10.4.3.16/28 10.4.4.32/27 10.4.5.64/26 10.4.6.128/25
The other present subnets are something like 10.7.x.x. I add this here just to see that it does not affect the result of this task.
The request is to configure a prefix-list following these rules:
- allow all subnets in 10.4.0.0
- as specific as possible, I should not allow other prefixes
- minimum number of prefix-lists
Considering the above one I did consider the following to be the correct answer:
ip prefix-list ONE permit 10.4.0.0/21 ge 25 le 30
However the proposed solution was:
ip prefix-list ONE seq 5 permit 10.4.0.0/22 ge 23 le 30 ip prefix-list ONE seq 10 permit 10.4.4.0/22 ge 23 le 28
Both solutions are working, nevertheless which one would be the correct one in the exam?
ip prefix-list ONE seq 5 permit 10.4.0.0/22 ge 28 le 30 ip prefix-list ONE seq 10 permit 10.4.4.0/22 ge 25 le 27
Also a good solution, but is the best solution? Hard to say. The advice that I got is that if you have such task in the lab exam to describe the issue to your proctor, explain all possible solution so he or she can see that you fully understand the multiple solutions and base on the answer pickup the best solution. I think this is the only way to solve this kind of questions. If you have other ideas, I would like to hear them.