OSPF Puzzle IV: LSA Discussion with Sample CCNP OSPF Questions and Polls

Even if you work in an OSPF shop, most people don't look OSPF database every day. Most people don't think about the cost of OSPF routes by thinking about what LSAs the SPF algorithm examines to find the cost of each route. What do people do? They design OSPF, configure it, and it generally works without a lot of attention.

The downside, of course, is that when it comes time to answer test questions, Cisco wants you to demonstrate mastery of the topic. Maybe you memorized the OSPF LSA types, and maybe you even really understand them. Maybe you used to know them well, and if need be, you can always open up a book for a quick 2 minute refresher. However, Cisco exams that look at OSPF - in particular, the CCNP routing exam and the CCIE R/S written exam - pose a few issues as compared to what people learn on the job:

  1. When most engineers think about OSPF costs, they calculate the cost by looking at a network diagram with costs on it - which is not what the OSPF SPF algorithm does.
  2. Many people have trouble what LSAs should exist in each area, so unless that info is supplied in the question, the question is difficult
  3. Even if supplied by the question, the wording of the headings in the "show ip ospf database..." commands does not list the LSA types - instead listing descriptions of the LSA types - making questions that use that output a bigger challenge.

So, how do you prepare for exams like CCNP routing, or CCIE R/S written, and overcome such challenges? First, I think it requires some application of LSA concepts to different scenarios, requires thinking about LSAs in those scenarios without the benefit of show commands, and it takes thinking about those same scenarios by just looking at the output of the show ip ospf database command. Maybe other activities would help as well - feel free to tell and others what's worked for you. But I do want to ask the question related to which is more difficult for you personally:

So, to answer the poll, you need to look at the questions below. Here's question 1, which requires more abstract thought about LSAs. This question is akin to something you might find in a CCNP prep book:

Assumptions:

The same configuration of OSPF and OSPF costs as seen in this series of blog posts, with no other OSPF-related config

The OSPF costs are based on the figure above

The following output on R1 and R2:

Click for Exhibit, Question 1

Question 1 requires abstract thought about LSAs, and consideration as to the area(s) in which the router exists. Next, try out this variation, which supplies the output of the "show ip ospf database" command on R1. (It's probably better that you answer Question 1 first, before looking at the output.) Question 2 is asking about conditions similar to question 1, answer A:

Click for Exhibit, Question 2

FYI, if you're getting into this, you could extrapolate Question 1 answers C and E into two similar questions, and with the "show ip ospf database" output from R2, make two more questions out of question 1's answers B and C. Just in case, here's a link for "show ip ospf database" on R2:

So, if you haven't already, go back up and answer the poll on which is the more difficult type of question for you. Also, share any tips or ideas you have for how to prep for OSPF, and any comments you have on the things that make OSPF questions difficult. As usual, please don't post what you think the answer is in the title of first few lines of any post, to avoid spoiling the answers. Thanks!

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