I have recently been researching anti-spoofing for two ISPs and have an operational question or two. We have found that about 5% of sites are connected to 2 ISPs but do not use BGP. They use two different sets of IP addresses and point default to one of the two ISPs. The return traffic will of course go to them via one of the two ISPs, but if you have anti-spoofing filters set up or try to set it up now, you will break their outgoing traffic. Reasons to allow it: 1) Here we have multihomed customers, who are not eating up ASN space and are not asking for PI space and are happy the way they are working. By stopping them, we will force this 5% to ask for ASNs and PI space. So for the general well-being of the Internet - why not just let them be. 2) Anti-spoofing is set up to stop attacks from unknown IPs (RFC1918) or from an IP that doesn't belong to you. In this case, the IP can be traced back to the user (via ISP #2). 3) If you block it, the customer will leave and go to another ISP that does not block these IP addresses. Reasons to not allow it: 1) If ISP #1 has blocked the customer due to being an open mail relay (example), and then that customer just sends the traffic out via ISP #2 (using ISP #1 IPs), they have circumvented the filter and blame will be placed on ISP #1 for not stopping an open email relay (this has actually happened once before). 2) I should not be announcing traffic for IPs that I am not announcing routing updates. I am curious if others have found this 5% occurence and I am curious why no one else has raised this issue before. Could it be that almost no one is running uRPF and/or anti-spoofing filters? Thanks, Hank