FW: /8s and filtering

Harsha Narayan hnarayan at cs.ucsd.edu
Tue Dec 10 19:32:03 UTC 2002

  Now I am confused because I have got two sets of contradicting answers.
Some say that anyone can multihome, some say that you need to be of a
certain minimum size to multihome. May I know what is the right answer?

  I agree that allowing anyone to multihome would increase the size of the
routing table. So does this mean that someone has to be of a certain size
to multihome?


On Tue, 10 Dec 2002, David Schwartz wrote:

> On Tue, 10 Dec 2002 12:36:39 -0600 (CST), Forrest wrote:
> >Maybe I'm missing something, but what good would it do for someone to
> >multihome if only their own providers accept their route, but nobody else
> >does?  I realize that their block should be still announced with their
> >ISP's larger aggregate, but what good does this do if your ISP goes down
> >and can't announce the large aggregate.
> 	Smaller multihomers elect to multihome for a variety of reasons. Those
> reasons typically include latency reduction and toleration of POP failures,
> router failures, and line failures. They're not looking to stay online is
> Sprint or MCI disappears entirely.
> 	If you multihomed to 2 providers in this manner and made a table of all your
> downtime and its causes, loss of the larger aggregate would the tiniest
> fraction of your downtime, which is already a tiny fraction of the time.
> 	We don't put parachutes on commuter jets. The failures where
> these would be helpful are but the tineiest fraction of the failures that
> occur.  And any significant failure at all of such a redundant system is
> rare.
> >If you're a smaller organization, perhaps you'll only have a /23 from your
> >upstream provider.  With the filtering that seems to be in place, it seems
> >like the only way you can truly multihome with a /23 is if it happens to
> >be in the old Class C space.  Or is this wrong?
> 	You're just biasing the question with the choice of words you use ...
> "truly" multihome.
> >What seems to be needed is perhaps a /8 set aside by the RIR specifically
> >to allocate to small organizations that wish to multihome that people
> >would accept /24 and shorter from.
> 	Not only would this increase the size of the global routing table, but this
> would actually decrease reliability for most basement multihomers. Basement
> multihomers tend to flap their routes more often than their upstreams. By not
> being inside a larger aggregate, these flaps are likely to result in more
> significant pockets of unreachability than they would be otherwise.
> 	DS

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