Sriram, Kotikalapudi (Fed) wrote on 08/03/2022 19:36:
This question has relevance to the ASPA method for route leak
detection.
Is it possible that an ISP AS A peers with a customer AS C via a
non-transparent IXP AS B?
IOW, the AS path in routes propagated by the ISP A for customer C's
prefixes looks like this: A B C.
I.e., can the AS of a non-transparent IXP/RS appear in an AS path in
the middle between an ISP and its customer?
Almost all RS's are transparent these days. Usually IXPs go to lengths
to ensure that the RS ASN doesn't appear in the AS path.
Some organisations provide transit over IXPs, but it's a minority thing.
It would be very peculiar if an organisation provided transit over an
IXP via an RS.
Some organisations provide transit to ASNs over a direct physical
connection while maintain peering with their customer over an IXP port.
Usually this happens by accident, but occasionally it can happen by design.
The answer to your question is that it would be technically possible,
but it would be so peculiar and stupid that it should be considered a
mistake in the situations where it was intentional. In all other
situations, it would be a leak. Generally it would be safe to assume
that this sort of configuration was in error.
Nick
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