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!
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).