"pretty rare", indeed, never heard of it being done, but with depletion of IPv4 addresses, I can see it as a way to dual home a customer.
Sorry for my confusion about source ASN.
If we only had one ISP, it would advertise its public IP address block(s) using its ASN. Once traffic to one of its advertised address blocks got to it, internally, it would route that traffic to the correct customer. (This is also how an ISP can hand out subnets of a public /24 to different customers.)
If a company wanted to use multiple ISP, in ye olden times, the customer would apply for an ASN and an address block(s), and advertise such address block(s) to whoever they publicly peered with.
The big difference, for the two prior approaches, in the first, the /24 would have the ISP ASN, in the second, the /24 would have the company's ASN.
In this case, the ISP, Zayo, should be the "owner" of the /24, and when seen on the Internet, you would expect to see the last ASN as Zayo's.
The easiest way, I think, to proceed, would be for Zayo to "sublet" the /24, i.e. allow it to appear to be coming from our OP's ASN (much as you would if OP actually "owned" the /24). Don't know all the ARIN rules, whether this is "kosher".
An alternative, would be, since the OP does have a public ASN, logically have it peer with Zayo and Comcast, providing "transit", but only for one particular /24. In this case, the final ASN for the /24 would still be Zayo's ASN. This approach, would be more complex to get "right", but the /24's ASN would be seen, on the Internet, with the final ASN as expected from ARIN info (unless they too now support "subletting"?).
Rich, is the forgoing making any sense? Basically, for such a "borrowed" public address block, what was the final path ASN for it in your case, do you recall?
