ISP Filter Policies--Effect is what?

Stephen J. Wilcox steve at
Tue May 8 16:35:21 UTC 2001

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.

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.


> 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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