two AS announce the same adress space

[Mikael Abrahamsson <swmike () swm pp se>]

What will break if two different ASes announce the exact same adress
space?

It's the old multihoming issue, small company wants to multihome, does not
want to get an AS of its own, and wants to use one address space via both
providers. This adress space is from PI space so ISP1 already announces
the exact net (no aggregation) and would like ISP2 to also announce this,
perhaps via a private peering using "private AS" but I would like to avoid
that.

I know the RFC says this should not be done (or at least I'm told so), but
I would like to know if someone does this anyway and if they do, what
problems arise?

-- 
Mikael Abrahamsson    email: swmike () swm pp se