Ethan Banks Not writing about IT.

NMC DOiT Vol.2 Scenario 5 Day 2 – General OSPF Comments + Making Things Harder Than They Are


Oh, man. I’m getting killed here. I’m way too tired to be doing this. My brain is just not with the program tonight. It’s not that the material is so hard as much as I’m just not thinking clearly. I’ve spent most of the last 2 hours hacking and slashing my way through basic interface set up and OSPF. The OSPF was a little painful in that you had to know what network types result in a network with a DR (or not), plus which ones require a neighbor statement (or not). And you also had to know what the default OSPF network type was going to be on a particular frame link. Fair enough…all things I should know off the top of my head, but I don’t quite yet. I’m getting there, but I still have to think about it. So I was doing a lot of “show ip ospf interface” to determine what the default type was, then racking my brain to remember whether that default would meet the scenario requirements, or whether I needed to tweak the interface type. But I got through it.

Then I did a real dumb thing. The scenario wanted 2 of the OSPF areas to have no external routing information, but be able to pass external routes into the rest of the OSPF domain if those external routes originated within the area. Okay, that would be an NSSA area. No type 5 LSAs allowed in, but it will generate type 7 LSAs and forward them to the ABR, who will forward them to the backbone area as type 5s. But for some reason I was thinking I had to do a “no summary”. The way I interpreted the scenario request, I thought I needed to clobber routes originated from other OSPF areas as well (LSA type 3s), as well as the type 5s. Well, the catch with the “no summary” command is that the ABR then generates a quad-zero default route and pumps it into the totally nssa area. And the scenario said that quad-zero routes are a no-no. So I’m trying to think of a way that at the ABR, I can filter out the quad-zero advertisement into the totally nssa area.

The solution to the problem is that I was reading too much into the question. I didn’t need to make it nssa area totally nssa. Just nssa was fine. A regular NSSA area has no quad-zero route, and routes external to OSPF will still be filtered off. That was the answer, simple as pie, and I was making into the weird problem, wasting time trying to solve an issue I invented. What I was trying to do was silly in retrospect, as I’d have ended up with no routes at all heading into the area. Sigh…

I know I’ve blogged it before, but I’m saying it again…this process is as much about learning HOW to take this exam as it is learning the technology. I’m calling an early night here. Going to catch a little tube time, and then get to bed early.

By Ethan Banks
Ethan Banks Not writing about IT.

You probably know Ethan Banks because he writes & podcasts about IT. This site is his, but covers other stuff.

Get the details on his about page.