I've read the current policy on ARIN's allocation of space and I must admit that I'm still confused. First, allow me to state the assumptions that I'm under. I understand the policy to state that if a business needs to multi-home and requires less space than a /20, then they should request this space from their ISP. I also understand that there are filters at the /20 boundaries in order to minimize the size of the routing table. Question: Doesn't this break multi-homing for end users that need less than a /20? For example, assume that the end user is connected to two regional ISPs (ISP-A and ISP-B). Neither of which have agreements with each other. However, they do share a common backbone with a national provider we will call ISP-Z. If ISP-Z has filters at /20 for both of the ISPs that it is connected to, then ISP-A address space will be the only space listened to on the ISP-A to ISP-Z link. The same would be true for the ISP-B address space only being listed on the ISP-B to ISP-Z link. This creates a situation where address space from ISP-B would not be advertised through ISP-A and in effect, breaks multi-homing. Consider a remote site attempting to reach the web server at the end user. DNS resolves the address to ISP-B address space. Also assume that the link between the end user and ISP-B is down. As the packet enters the national carrier ISP-Z's network, at some point the router will have to decide to send the pack on. If ISP-B is still advertising the remaining portion of their network (say at the /20 boundary) then ISP-Z will forward the packet to ISP-B. This is normal and proper for a single homed address space. However, if the end user had their own micro allocation, their address space would be advertised to both ISP-A and to ISP-B and in turn to the national carrier. As such, the destination network route would be dropped from the advertisement coming out of ISP-B and the only remaining route would be via ISP-A and the packet would still get there - if the end user had a micro allocation as per previous policy. Also, let us further look a situation where ISP-B is down. When the national carrier detects ISP-B is down it will remove that particular route from it's table. In the old way of doing things with micro allocation to multi-homed end users, ISP-A would advertise the address space from the end user. It is my understanding that under the current policy, ISP-A would have to advertise the address space allocated to the end user from ISP-B. If the address is less than a /20 and if the national carrier is filtering on a /20, wouldn't that cause the update to be dropped and thus not added to the routing table for the national carrier? I guess my confusion could be cleared up if someone could describe how, under the /20 policy, an end user requiring multi-homing and less than a /20 allocation would be able to survive one of their two ISPs going down (remember the AT&T and MCI outages?). Thanks, PS. I have sent this email to the ARIN policy list but have not received any useful responses. -- David Lott VP of Operations MSN Communications (303) 347-8303