Re: Advertising rented IPv4 prefix from a different ASN.

[Chris Welti <chris.welti () switch ch>]

I would not recommend to do that.

If you really do this, please make sure that the owner of the supernet (in this case the university) also does transit 
for the subnet (which they should as they are supposed to accept and forward traffic for the whole aggregate that they 
are announcing).

Otherwise, for networks that only do partial routing (basically defaults from transits + peering routes), this will 
create a blackhole in case they peer with the ISP that announces only the supernet,
but not with the ISP that announces the subnet, because traffic will always be routed towards the announcement of the 
supernet only.
Same applies if the subnet gets filtered by some people for policy reasons (like no more-specifics of PA space, or 
smaller than /24...).

Also, be careful that the owner of the supernet doesn't apply inbound anti-spoofing filters at their borders towards 
transits and peers for traffic from your subnet that is part of their supernet.

Chris

On 04/08/16 21:39, Andrew wrote:
> Hello List,
>
> I work for a medium sized ISP.  We are entering an agreement to rent
> some IPv4 space from a local higher education institution.  Being a
> multi-homed ISP we would like to advertise the rented prefix from our
> ASN.  The prefix that will be advertised is a smaller subnet from the
> higher educations block; they will continue to advertise the larger
> prefix.
>
> What is the best way to accomplish this?  Is there any way of doing this
> without having to tunnel the traffic through the origin ASN?
>
> I feel if we just adverse the prefix it get put on a bogon list for
> prefix hijacking.  This space is rented long term but they are not
> interested in reassigning the space to us.  They also want to keep
> advertising their prefix as one contiguous block.
>
> I appreciate any insight and information.
> Thank you for your time,
> Andrew.