[c-nsp] BGP route filtering question about upstreams

[Andrew (Andy) Ashley]

Hi all,

I¹m hoping someone can provide a bit of insight here with a BGP route
filtering scenario:

AS100 is an end-customer stub AS, multi-homed to upstreams AS200 and AS300.
AS200 also buys transit from AS300, amongst others.

AS100 does not want AS300 to learn its routes from AS200, since that can
cause redundancy issues (2 supposedly diverse upstreams effectively become
1).
AS100 still wants to receive a full table from AS200 (but not routes that
transit AS300).

AS100 asks AS200 to filter its announcements to AS300.
AS100 now still receives routes that are learned from AS300 via AS200.

It should be possible for AS200 to tag prefixes learned from AS300 at
ingress, then implement a policy to filter these tagged prefixes on outbound
announcements to AS100.
But, how can AS100 still receive a full table from AS200 with such filtering
in place?

Thank you!


Regards,

Andrew Ashley