This discussion is basically correct. Some comments: 1) You always want to advertise the entire block (in this case, a /16). If you don't own the entire block, then you should have a relationship with to BGP originator of the block advertisement such that you can receive traffic from them where you expect it (if, for example, you AS is not really "well connected".) 2) There is no substitute for looking at things in the real world. To understand how filtering works and routes propagate, I suggest two resources: University of Oregon Route Views Project http://antc.uoregon.edu/route-views telnet route-views.oregon-ix.net The route-views.oregon-ix.net router receives BGP from all but a few of the major backbones and an assortment of others as well. You can query that router to see who hears what routes. I was able to verify, for example that Verio filters all Class B subnets (including /18, for example). Traceloop (I am affiliated) http://www.traceloop.com The Traceloop network is basically a searchable community of traceroute servers. You'll have to sign up to get a personal login, since the Guest Login doesn't have all the features that you'll want. If you type "AS2914" in the search field (for example), you'll get a list of test points in Verio's AS (although the formatting is poor). If you type "www.verio.net" in the search field, you'll get a list of test points in Verio's AS and some that use Verio for outbound routes, too. By choosing test points in various AS's, you can see how traffic is actually routed. -steve Dashbit -- The Leader In Internet Topology www.dashbit.com www.traceloop.com On Tue, 8 May 2001, Murphy, Brennan wrote:
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