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Our own or our singlehomed customers' address space -- we would reject such an advertisement. The same inbound consistency check applies to peers and upstreams/transits.
If it's someone else's or a more specific or the same prefix as our multihomed customers -- we accept it. There isn't anything else we can do in practise which would not hurt legitimate routing.. <clip>
What do you do when one of your multi-homed customers on your IP space has an outage on their connection to your network? How would your customers then reach that customer? Although this wouldn't be THAT BIG of a deal for small networks, if say a larger or a Tier-1 provider practiced this (AFAIK, the only somewhat large network to do this is, believe it or not, PCCW), your customer would experience a major outage. There must be a better way. :)
Pekka Savola
Regards, Randy