Thank you to all who replied for your consideration, and thank you to Acee for the kind welcome!
In regards the idea to change the wording to "The IS-IS FAD Sub-TLV has an area/level scope", I personally think that any mention of the word "area" in regards to IS-IS flooding could still be a source of confusion.
I'll expand the example in my previous mail, in case it's helpful. Imagine a theoretical level 2 topology which contains a few hundred routers.
- Some routers are in area 49.0001
- Some routers are in area 49.0002
- Some routers are in area 49.0003
- Some routers are in area 49.0004
(I know "49" is not strictly speaking part of the area identifier, but I've included it in the example just for clarity.)
If a router in area 49.0002 were to generate the IS-IS FAD Sub-TLV, I believe the intended delivery scope is "all routers in the same level as the original sender", regardless of the area the router is in. It's the level that defines the flooding scope, not the area. So for example, L2 routers in area 49.0004 should receive this sub-TLV,
Even if we were to talk about level 1, it is possible for an L1 router to be in two IS-IS areas at once, which is a way of creating a single L1 topology, a single LSP flooding domain.
With all that in mind, hopefully it's a bit clearer why I worry about any mention of the word "area" in IS-IS when it comes to describing flooding scope, and why I feel that the wording "has an area/level scope" still has the potential to cause confusion. As a reader, I would wonder whether the implementer has a choice in the scope. The intention would not be explicitly clear to me. The word "area" has a slightly different meaning in IS-IS than it does in OSPF.
Hopefully that explanation is helpful. I'm very aware that I'm a newcomer talking to people far more knowledgeable than me about things like this, so I hope you'll forgive me if it turns out I'm mistaken.
All the best
Chris