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.
Further, there will be no "intervening router" in this case, since
each of R1, 2, and 3 will be adjacent to only N1. 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. 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.
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.)
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.
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