Address portability all depends on if you IP blocks are assigned by
ARIN/RIPE/APNIC/ISP portable or if you are using the ISP's address
space.
It has been my experience that multi-homing to diverse ISP's
with multiple circuits per ISP (i.e. Primary/Secondary with ISP-A and
Primary/Secondary with ISP-B) is the best option if you can afford the
cost and your bandwidth requires it.
Like it was stated before, if you can afford the possible
downtime associated with multi-homing to a single ISP then yes there are
definitely cost savings to be had and reduced administrative overhead;
but, if you cannot afford the possibility of downtime then separate
ISP's is the only way to go.
Chris Burton
Network Engineer
Walt Disney Internet Group: Network Services
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