Section 2.1 Task question

One of the tasks is

  • Ensure that default-routes generated by OSPF are only advertised into Area 121.

I eventually had to look up the answer after not being entirely sure what is even being requested:  Are they wanting 121 to be totally stub/nssa?  Or filtering default routes from area X out of everywhere but 121?  Either way the wording is vague and doesn't really fit.

And I find that I'm supposed to infer from that that

1) Area 121 should be NSSA

2) Redistribution on R16 should not have the P-bit on, causing R1 and R2 to not convert the Type-7 to Type-5.


I am not at all sure how I am supposed to be able to infer that.

You could argue that the EIGRP domain on the other side of R16 suggests it, and perhaps this is why you would read the entire Lab before starting (which is also something I still can't wrap my head around -- remembering to account for task 97 that you read when you're implementing task 3)

But the second part?  How in the world does the instructions possibly lead to that?  Can anyone explain how to get from A to B with that?


  • Yeah; that's the HOW.  But you have to know the WHAT before you can determine the HOW

  • Here are the first to bullet points of section 1.2:

    * Configure Area 121 to block Type-5 LSAs from entering the area, but allow R16 to receive Type-3 LSAs from other areas. Don’t use routing filters to accomplish this.

    * Ensure that default-routes generated by OSPF are only advertised into Area 121.


    Here is the very last bullet point section 1.2:


    * Redistribute the Loopback0 interface of R16 into OSPF. 


    The first and last bullet point are hinting at using NSSA for Area 121 - Block Type-5 LSAs, but allow Type-3s, and allow R16 to do redistribution of its loopback. NSSA is the way to go.


    * Ensure that default-routes generated by OSPF are only advertised into Area 121.

    Before moving on to section 2, task 1.6 asks for the following:


    ## 1.6    Redistribution - Core Routing 

    * Ensure full and stable reachability between all the routers in the OSPF, RIP, and EIGRP domains - R1-R8, and R16. 


    This is where the two are tied together. Up to section 1.6, routing in the core network should have all been configured. The only missing peice is giving R16 a way out of the NSSA. The easy way to accomplish this, which falls within the scope of what the task is asking, is to originate a default route on Area 121 ABRs towards R16. By default this default route originated by the NSSA ABRs has the P-bit cleared, so it will not propagate further into other areas (if there were other ABRs on Area 121 for example).

    The wording of the question is guiding you to ONLY originate a default route into Area 121 - which is the only place that is needed in order to obtain full reachability within the core.



  • That's what I originally though too; to have the ABRs (R1 & R2) originate a default-route ONLY (IE make the nssa area a T-nssa area), but I thought I'd read that the solution refers to default routes originated from R16 not being converted (7-to-5) by one of the ABRs for the rest of the domain.

    I'm in the process of rebuilding the IOU lab on my home server; I have a setup at work I was using but I can't access it except when I'm at work.  Once it's setup I'll start & review if I'm mistaken in what the solution showed. :)

  • I was mistaken in how I'd read the solution previously.  Thanks for the detailed explanation plucena!

  • After tinkering around with the nssa-only keyword from the solution, I find it's not actually needed with the 'area 121 nssa default-information-originate' command.  Even if only 1 of the ABRs puts a DR into area 121, it won't leak through the other ABR into the rest of the domain anyway.

    It looks to only used when an ASBR inside the nssa is redistributing a DR into the nssa and you don't want it converted by the ABRs.

    Verified here.  It's good when my thought processes are correct!

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