ISP Filter Policies--Effect is what?

john heasley heas at
Tue May 8 17:28:01 UTC 2001

Tue, May 08, 2001 at 05:35:21PM +0100, Stephen J. Wilcox:
> Cant comment on verios policys but...
> > 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.
> if you have a /16 why would it be broken down to /24? i would assume the
> only reason you advertise /24 is because that is the size of your
> assignment from the NIC, in which case you cannot advertise the /16.
> if you do own the /16 then yes of course you can advertise it.


> > 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? 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?
> maybe, see above, if they cant advertise the /16 then theres no route. if
> its a verio customer then if verio dont advertise the /24 then no bgp will
> propogate and no routes will be valid.

no allocations have been made by RIRs in B space longer than /16, so they
have the /16 to announce.

> dont forget for traffic going TO the customer the traffic needs to find
> verio first and then the next hop will be the customer so verio wont pass
> it to anyone.
> if it were possible to advertise both via different providers and the
> other provider accepted the /24 then the verio routes will be only used
> where the /24s dont propogate so there will be a shift in traffic to the
> other provider, if these are both transits so the routes are advertised
> out then your not going to get any traffic giong over verio.
> I think you're forgetting that inbound packets and outbound packets are
> independent in finding their way through a network, a bidirectional flow
> does not mean that in and out go the same way.. 
> in your example above you'll have packets going out nicely balanced but
> coming back in will just be through the /24 acceptor and not verio.
> Steve
> > 
> > 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
> > 
> > 
> > 
> > 
> > 
> > 
> -- 
> Stephen J. Wilcox
> IP Services Manager, Opal Telecom
> Tel: 0161 222 2000
> Fax: 0161 222 2008

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