Dave Katz <[email protected]> wrote on 07/08/2009 00:04:59: > > > On Aug 6, 2009, at 1:28 PM, Joakim Tjernlund wrote: > > > Dave Katz <[email protected]> wrote on 06/08/2009 20:24:25: > > > > Hi Dave :) > > > >> > >> If there is no back link, there is no link, and SPF moves on. If the > >> network is partitioned, somebody should fix it. > > > > Yes, this problem exist for a short while, until the remote > > route has updated it's LSA. > > > >> > >> Ignoring the bidirectional test rule can lead to loops and black > >> holes, particularly if other implementations are following the rules. > > > > hmm, can you be more specfic? > > > > Consider this example: > > > > R1 R2 R3 > > | | | > > ------- N1 > > > > Destination is R3 and R2 is the calculation router. R2 cannot find a > > backlink > > so it falls back to the intervening router case and uses the nexthops > > from the "parent", hoping that one of them will redirect IP frames to > > R3. > > This is an uninteresting case, as there are no alternative paths.
Yes, it is very simple to make it clear what I am after. > Further, there will be no "intervening router" in this case, since > each of R1, 2, and 3 will be adjacent to only N1. Yes, I am just saying that in this particular case I want apply the same SPF rules as the intervening router uses. > There will be no > next hops to inherit. The "intervening router" rule basically just > points out that if the best path from A to C is through B, the next > hop to C is simply the next hop to B. More or less. > > Where it can get much uglier is if there is a back path between R2 and > R3; other routers playing properly will try to send the traffic > through R3, but R3 will try to forward it locally. hmm, don't you mean "through R2, but R2" ? > In this case it > will probably result in the traffic just disappearing down a hole > instead of being properly delivered on the alternative path. But if > you start playing these games more generally, you can end up with > traffic flying in circles. I am starting to think that my idea was a bad idea now :) > > While routing is transiently nondeterministic, "hoping" is a bit less > rigorous than required. ;-) And by "redirect" I assume you mean > "forward", as "redirect" means something specific (and only applies to > hosts.) Yes, I meant forwarding(sloppy me :) > > In the general case, routing only converges on stability because > everyone plays the game the same way, and the rules are designed to > make it happen. If you change the rules or somebody plays them > differently, it is very easy to break the algorithm. > > It's not at all clear to me why this would be interesting, even if it > worked. In a modern, competent OSPF implementation (namely, one which > ignores the rules about repeated LSA generation and is otherwise > carefully tuned) you will see the other guy's LSA with the back link > within a few milliseconds anyhow. In a well-engineered network there > will be an alternative path anyhow, and traffic can be delivered on > that path until the new one comes up fully. > > _______________________________________________ OSPF mailing list [email protected] https://www.ietf.org/mailman/listinfo/ospf
