Brian E Carpenter allegedly wrote on 03 21 2009 4:07 PM:
So instead, you run NAT at every ISP connection. Your internal users get
NATted to an ISP prefix at whichever exit point their traffic happens
to reach, which automatically causes their return traffic to come through
the same ISP. That exit point is locally chosen by the local routing setup.
You don't need any worldwide coordination of the BGP4 advertisements,
because there aren't any expect the ISP's normal ones. Also, traffic
flows inside your network are localised, since traffic goes out and
returns through a (reasonably) local gateway.
When one of these NATs goes down, active connections will be lost,
but IGP routing will switch users automatically to a different NAT
when they retry.
If you allow your hosts to use multiple connection points into the
Internet, and external routing changes so that the packets they send go
out different connection points, their apparent source address can
change. One of the requirements for effective use of NAT and
multihoming is that your hosts' peers need to handle this (via
Multipath, HIP, MIP, SCTP or whatever). That is, you can't allow your
hosts to use multiple connection points until everyone _else_ they talk
to has been upgraded. How will you know when that is?
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