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Re: Failover for a multihomed site with unreachable ISP
On donderdag, maa 27, 2003, at 08:14 Europe/Amsterdam, Christian Schild
1. An ISP's aggregate route (the /32) disappearing from the global
routing table is an extremely rare event. Last mile problems are
infinitely more common,
Maybe a better solution would be a backup organization that announces
the /32 if it disappears and tunnels the packets to the customers over
the secondary ISPs.
Well, rare is not zero :). We just want to cover that case.
and after that there is the class of partial ISP failures (for
a single POP is partitioned from the network) where the aggregate
Hm, if just one POP disappears, the ISP (and the customer) is still
??? If the POP the customer connects to goes down, the customer isn't
reachable over the addresses from that ISP anymore.
we don't have a loss of connectivity and no action has to be taken.
2. Conditional announcement makes the global routing system less
predictable, which is dangerous. The sudden appearance of thousands
specific /48s could easily break systems that could just about cope
regular aggregated situation.
I see. But in our approach 2, the longer prefix is already present in
routing table. Only if the shorter prefix disappears, it will be added
forwarding table. It is some kind of 'automatic aggregation'. Do you
will be that extremely costly?
Not costly, dangerous.
If the routers must be able to handle the full set of unaggregated
prefixes anyway, why bother aggregating? Maybe this can work in theory
but not in practice. Many people will just filter the /48s to avoid
Moreover I think, systems 'that could just
about cope' should be replaced, and they shouldn't be a concern here.