ISP Filter Policies--Effect is what?

Kevin Gannon kevin at
Tue May 8 16:24:53 UTC 2001

>Site                    BGP Advertisement       to      ISP
>Amsterdam              AMSISP
>Austin         Genuity & Internap
>SanFran         Genuity & Internap
>Tokyo                  TOKISP
>Sydney         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
>that wants to extend connectivity to verio customers (and any
>other ISP with similar policies) must include a /16 advertisement from at
>one of the sites.
>2. Suppose a customer of a Verio-like ISP, wishes to go to ftp. DNS
>returns (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?
>means, a path exists for that user to the amsterdam server and the only
>with routing to Amsterdam is that Verio possibly handed the traffic to a
>neighbor. Am I understanding this issue correctly?

Yes the logic seems clean , also another situation which might
happen is that Verio peers with ISP x and ISP x doesnt have a
specific /24 route they will follow a static to somewhere. You
want to be careful that the somewhere knows about the /24.
This could easily happen if Verio peers with the ISP that is
nearer and an ISP that also peers with this ISP. So verio
might decide hey the "longer" path is better and the packet
gets into the "longer" ISP and it wont have a /24 so it will
follow the /16.

Maybe if you could tell use who owns the addresses and how
big the original block is ? A whois points to a swiss bank is
this correct ?

>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
>each of them had this vague memory of Verio's policy but couldnt really tell
>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.

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