A few points regarding flooding reduction algorithms. Leaderless. === I prefer a distributed flooding algorithm over a centralized flooding algorithm. But I do not prefer the leaderless way. It will be hard already to make 2 different flooding algorithms in the same network cooperate. (legacy and a new one). Having multiple algorithms, and not knowing what the other routers do, will be even harder. Having a leader is very much like having routers advertise the FAD in flex-algo/ (This analogy comes from Peter). It ensures everyone runs the same FA algorithm. And if a router doesn't understand the FAD, it falls back to default behaviour (and just doesn't participate in this flex-algo). Having something similar for the flooding algorithm too will make things simpler. And thus more robust.
Flooding topology repair. === The hard part of optimized flooding is not coming up with different algorithms to prune links. The hard part is to repair the flooding topology when it breaks. If I understand correctly, the only real mechanism we have for that is sending CSNPs on p2p links? CSNPs are not reliable. So once you use them, you need to keep sending periodic CSNPs forever. Imho when you have to do that at scale, that solution is worse than the original problem we were trying to solve. Flooding reduction when an IS-IS process (re)starts. === Imho, the biggest scalability issue with flooding is when an IS-IS process, in a large network, (re)starts. That is when potentially all LSPs are sent over all adjacencies. No other event will cause this much (local) flooding. But unfortunately, when IS-IS starts, it has an empty LSPDB. And thus it has no idea about the topology yet. So it can not really participate in any flooding reduction algorithm (yet). Only when it has received all or most LSPs, it can participate in any flooding reduction algorithm. But by then, the need for it has already diminished. So in any proposed flooding reduction algorithm, I would like to see these two points discussed in detail. 1) How to repair a broken flooding topology ? (Preferably without periodic CSNPs). And 2) How beneficial is the algorithm during process (re)start? henk
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