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The latest, highly technical post from Petr on OSPF!

Challenge solutions. Comments / submitted solutions are now available on the original post.

Followup to "Have you seen my Router ID?", dissecting a task from a strategy / knowledge point of view.

Challenge post on OSPF configuration.

The feature we are going to talk about today may look a bit convoluted, but it demonstrates core OSPF behavior: combining link-state and distance-vector behaviors. The command capability transit was introduced in IOS 12.3T and is on by default. However, the description is rather confusing and does not explain the underlying mechanics. We are going [...]

Intro There was a lot of blogging related to OSPF topics recently. In this post, I would like to clarify some common misunderstandings that many people have about OSPF route filtering. I have seen so many folks (some of them really experienced persons!) incorrectly understanding the underlying behavior so it’s about time to make this clear. [...]

The final in a series of blog posts examines the Totally Not-So-Stubby Area.

The second part in a series on OSPF area types.

The first in a blog series about OSPF areas.

Does an OSFP stub explicitly filter Type-4 LSAs, or is their absence in an OSPF stub area simply due to being unnecessary because the Type-5 LSAs have been filtered?

Highlight for answer: Actually, I don’t know the answer to this question.  I was thinking about it today.  In an OSFP stub area Type-5 LSAs are explicitly filtered.  There are no Type-4 LSAs present either.  I don’t know if they are explicitly filtered, or they are just never generated because the Type 5 LSA is filtered/never created?  It’s my understanding (possibly a misunderstanding) that the ASBR generates the Type-4 LSA, so…it must be explicity filtered at the ABR, right? <–This is WRONG!

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Thank you for the comments (big ups to Ivan P, Zeeshan, and Pavel Sefanov).  I think that I have this cleared up in my head now:

The ABR generates the Type-4 LSA. If the area is configured as a stub area, the ABR filters the Type-5 LSAs(generated by the ASBR) and does not generate a Type-4 LSA. So, technically, an OSPF stub configuration only explicitly filters Type-5 LSAs, but it implicitly filters Type-4 LSAs as well as there is no need for the ABR to generate a Type-4 LSA.

So if you were to tell a co-worker that both Type-5 and Type-4 LSAs are filtered, you would be technically wrong. 

Ivan Pepelnjak from Cisco IOS Hints and Tricks wrapped it up nicely:

To make it more explicit: the type-4 LSA is the glue that ties together a type-5 LSA originated by an out-of-area ASBR with the ABR flooding type-5 into the area. If there are no type-5 LSAs, type-4 LSAs are not needed (you will also not see them for ASBRs in the same area).