Hello,
wrote:
So in this instance, when you configure this, it seems to send it to the broadcast address
Incorrect. The FA in Type-5 LSA is the same IP@ that is configured as static route' next-hop when this static is exported into OSPF. The "broadcast" in the knob name "forwarding-address-to-broadcast" is referring to OSPF broadcast adjacency, not LAN broadcast IP.
wrote: what if you wanted to manually configure the address? I'm assuming that's not possible
It is possible. Set the static' route next-hop to an IP@ covered by subnet/netmask of OSPF broadcast adjacency link (say, 198.51.100.128/29 subnet with OSPF broadcast adjacency + static route 203.0.113.0/24 next-hop 198.51.100.133), export this static into OSPF and voila, Your FA would be set to 198.51.100.133.
wrote:
An example of when this behaviour is used was if two routers were facing the same broadcast network and an external next hop was reachable via that same broadcast network, it would make more sense to send it directly to the next hop, rather than the router that originated the external route. The example is here.
The authoritative example is in RFC 2328 section 2.3, and it also gives 2 use cases for FA. The second use case is dangerous though as it implies redistribution of BGP into OSPF which is not a good practice at all in 2020.
HTH
Thx
Alex