I don't think filters are a problem for third party routing as long as the third party routing is not done in secret. If I am sending you third party routes for someone, and you know it because I tell you I am and you agree to let me, then you can open your filters to the source port for those routes. Third party routes that are being done without the knowledge of the traffic target are a bad thing and shouldn't be done anyway...
In the third party routes scenario you describe above it is not just an issue of you telling me that you are sending my third party routes and me opening my filters. You must also tell the third party that you are sending me their routes. Then they can open their filters to allow my traffic. All in all I see this as a huge complication in the underlying structure of the Mae's. I would be detrimental to the interconnects if ISP's used filtering as a general practice. It is a great feature to have as a way to stop an abuse that has been detected and maybe even as a punishment for abusive ISP's, but lets leave out the extra layer of complication unless it really adds something to the network Scott Blandford IBM Global Network