On Thu, 24 May 2001, Albert Meyer wrote:

One of my ISP clients is leaving, and the person who was assigning IP's when they became our client chose to give them class C's scattered throughout our block. These are Sprint IP's which are assigned to us with our Sprint circuit. Our former client is getting a Sprint T1 and asking Sprint to route these class C's to them rather than us, and they tell me that Sprint appears willing. They don't have an AS, but they're getting another local ISP to advertise the IP's for them. I normally wouldn't agree to participate in such a mess, but we're shutting off the Sprint circuit in a couple of months, and I can't see making them re-IP hundreds of domains under the circumstances. Has anyone done something like this? I'm wondering how much it will increase the CPU load on my router. I'm already running at 20% average, and if it gets much over that I start dropping ICMP packets. I know enough BGP to stop advertising the appropriate class C's, but I'm not sure that this won't cause problems that I haven't considered. Will anyone refuse to accept advertisements which send adjacent /24's to different places? Is this an officially "broken" setup, or is it just ugly?

Just ugly, and as others here will no doubt point out, unfriendly to the size of Internet routing tables everywhere. If you really want to do this, you should keep anouncing your whole IP address block, since the /24s will get filtered in some other places. Your former customer (if they really want to do this) should be anouncing the /24s, which will be more specific and will thus send traffic bound for their space to them. You will need to open your filters to let those announcements in, or else you won't be able to communicate with your former customer. This will work reliably only because you share an upstream provider, who will presumably be passing on your announcement of your shorter prefix to the rest of the Net, and who can presumably be paid to listen to the /24 anouncements. A further complication may arise if you have another upstream, who either isn't listening to the /24 announcements or has peers who aren't listening to them. Traffic to your fomer customer could end up taking a rather roundabout route, either through your other upstream or through your network. That said, just because you can do something doesn't mean you should. The weird routing scenario I described above (which depending on who your other upstreams are may not happen) would not only cost you money for bandwidth to carry traffic that you presumably aren't being paid for anymore, but would also have performance issues and may run into anti-spoofing filters, which would have to be modified. Growing the size of the Internet routing table, or at least the routing table as seen by those who don't filter out /24s, is a good thing to avoid when you can easily do so. IP renumbering is a big pain, but it's unfortunately a normal part of switching upstream providers for those without portable IP space. Are you big enough to get a portable IP address block from ARIN? Since you say you're dropping your Sprint connection in a few months, you will presumably have to renumber then. If you want to be really nice, you could bite the bullet and do it now, turning over the old block to your former customer. Alternatively, it's common practice to tell your customer that they have to renumber out of your space when they stop buying connectivity from you, and from your perspective that's probably the easiest way to handle the situation. -Steve -------------------------------------------------------------------------------- Steve Gibbard scg@gibbard.org