I wanted to follow up on my query below. I found a real-world case where a company advertises /16, /19 and /24 nets from the same Class B space....yet all are reachable from Verio's network (I tested this with Verio's Traceroute Server). A phone call to Verio verified for me why all 3 nets are reachable. (interestingly, the traceroutes themselves sort of masked how this was working--hence, the need to talk to Verio) Basically, it was as I had thought--because at least one of the sites advertises /16, this is enough to get traffic off of Verio's net, ie, all traffic destined for any piece of the /16 is passed to its "nearest" neighbor. If that neighbor in turn has the /19 & /24 in addition to the /16, the routing from that point on will be optimal. If not, it passes it to the neighbor from which it received the /16...and so on, until someone has the full /19 or /24 net. So, when I originally posted the note below, I was worried that Verio customers might not be able to reach certain networks with prefixes longer than /16, ie, sort of blackholing the routes to those destinations. (Remember, I'm new to Internet routing.) That would indeed seem to be the case if there were no /16 advertisement in addition to the more specific announcements. That's the generic BGP/Verio lesson I drew. The more specific lesson is that if your working with a large provider like Internap or Genuity in all your sites, traffic destined for your location is going to get back to you as long as they are advertising the /16 in addition to the more specific nets. ...even if the source hosts are on networks like Verio's. Any further comments appreciated. -----Original Message----- From: Murphy, Brennan [mailto:Brennan_Murphy@NAI.com] Sent: Tuesday, May 08, 2001 12:07 PM To: 'nanog@merit.edu' Subject: ISP Filter Policies--Effect is what? I'm trying to figure out to what degree the existence of these policies should be accounted for in a BGP design which includes sites around the world. I've read through a few of the threads having to do with Verio's Filtering Policy. And I read the policies listed here: http://www.nanog.org/filter.html Consider the following theoretical scenario: Site BGP Advertisement to ISP Amsterdam 169.61.201.0/24 AMSISP Austin 169.61.111.0/24 Genuity & Internap SanFran 169.61.119.0/24 Genuity & Internap Tokyo 169.61.202.0/24 TOKISP Sydney 169.61.156.0/24 SYDISP 1. Since Verio says they would not accept /24 nets drawn from Class B space, I assume this means that they don't insert a /16 into their tables so that the /24 nets appear to Verio customers as unreachable. In this case, a design that wants to extend connectivity to verio customers (and any other ISP with similar policies) must include a /16 advertisement from at least one of the sites. 2. Suppose a customer of a Verio-like ISP, wishes to go to ftp. foo.org. DNS returns 169.61.201.155 (in amsterdam, see above). Verio passes the traffic to the neighbor it received the /16 advertisement from. At this point, the best thing that could happen is if that neighbor has the /16 and /24 networks in its route table, right? That means, a path exists for that user to the amsterdam server and the only problem with routing to Amsterdam is that Verio possibly handed the traffic to a sub-optimal neighbor. Am I understanding this issue correctly? I'm new to BGP. I've tried to get a handle on this issue on my own and by working with Genuity, Internap and Cisco. No disrespect to those companies but each of them had this vague memory of Verio's policy but couldnt really tell me in plain language how it might affect the above scenario. Obviously, I wasn't talking to chief engineers. Someone from the CCIE mailing list suggested I browse the archives of this list, which I did. But I didnt find a clear enough answer to my questions--perhaps because they are too basic to be discussed here or I'm not good at using this lists archive search engine. Either way, any guidance on the above scenario is greatly appreciated. -BM