There may be some truth in both sides.
As part of a multihoming strategy, it is perfectly reasonable for ISP
1 to advertise space associated with ISP2, when the space being
advertised is a subset of the ISP 2's allocation, and ISP 1 learns
about the prefix(es) from a mutual customer.
It is also perfectly reasonable that ISP 1 would not accept the
advertisement, or possibly accept it but not export it, unless there
is some coordination between ISP 1 and ISP 2 that ISP 2 accepts the
idea of part of its space being advertised. This might be
coordinated by personal contact of routing engineers in both ISPs
(and the customer if the customer understands routing at this level),
or by having all parties register consistent routing policies in the
routing registry system.
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