Hello, everyone.
I have a quick question regarding OSPF’s FA. This topic is pretty confusing at times.
When it comes to type 7 LSAs, FA makes perfect sense.
Type 5 LSAs are the ones that confuse me, especially the requirements. In order for the FA to be put to any use, we need to have a scenario like this:
This is the weird part for me. Every resource has this - one router redistributing routes while also being connected to a segment with another router.
They use this example because it makes FA work. Without FA, R1 would calculate the route and use ASBR as the nexthop which wouldn’t be the most direct path.
To get to the point, there are some requirements that I don’t understand:
- Why does the interface on the ASBR need to be OSPF-enabled? It just feels weird, it connects to an external domain yet it has to be enabled for OSPF
- The rule above doesn’t say that an adjacency needs to exist there but the interface cannot be passive. This feels counterintuitive..
Why these two requirements? Why can’t the ASBR set the FA if these two aren’t met? If the ASBR wasn’t running OSPF on the LAN segment, it could still set the FA for everyone else and it would all work, wouldn’t it?
Also, I understand that they could ask about this on the exam and present such a scenario. But in the real world, have you ever seen a scenario of a shared segment in which only one router performs redistribution with its interface enabled in OSPF towards the external domain? 
It feels… unusual
Thank you!
David