On Wed, 13 Jun 2001, Michael Martin wrote:

The relevant portion of Tony's explanation (which is very concise) is the following --

++> but should see the route via P2 if P1 is accepting it. (Some ++> may either block the announcement or have anti-spoofing packet filters ++> at their borders that block the traffic itself).

His explanation is very good but the statement that Seth made was that many providers DO block the /24 announcements. Tony doesn't say anything specifically about this.

So the example is that you're numbered out of Provider1's CIDR block. You're fearing that Provider1 will block announcements of more-specifics from w/in their own blocks. My anecdotal understanding (which I agree has limited value) was that providers who filtered made *exceptions* in their filtering policies for their own CIDR blocks. *** What policies any other providers have is unimportant to my example and things will work just fine no matter the case with those people. *** At any rate, since you're a paying customer of P1, you at least have some influence to exert to get them to make exceptions. As for packet filters (vs. route filters), I doubt many ISPs would implement such a thing as that filtering is typically done at the customer edge. Tony

I remember plenty of threads on this topic but very few non-anecdotal facts about ISP filter policies. Not being with an ISP I'm very curious if there is a good answer. I'm not immediately impacted since Nortel has a class A to work with but I've run into this question from clients while doing consulting and just don't feel qualified to really answer it authoritatively. Anyone?

--- Michael Martin Internet Design Engineer Internet Engineering Nortel Networks