Splitting a block of Class C's

David Schwartz davids at webmaster.com
Fri May 25 05:11:59 UTC 2001

> This will work reliably only because you share an upstream provider, who
> will presumably be passing on your announcement of your shorter prefix to
> the rest of the Net, and who can presumably be paid to listen to the /24
> anouncements.  A further complication may arise if you have another
> upstream, who either isn't listening to the /24 announcements or has peers
> who aren't listening to them.  Traffic to your fomer customer could end up
> taking a rather roundabout route, either through your other upstream or
> through your network.

	If you are in the case where some of your former customer's traffic may
pass through your network, you are entirely justified in charging the
customer a fee for this 'service'. You may get some of his inbound traffic
unless all of your upstreams accept his /24s and meet with Sprint directly.

> The weird routing scenario I described above (which depending on who your
> other upstreams are may not happen) would not only cost you money for
> bandwidth to carry traffic that you presumably aren't being paid for
> anymore, but would also have performance issues and may run into
> anti-spoofing filters, which would have to be modified.

	The problem is, if your Sprint connection isn't live, you will have no sure
way to get his traffic to him, and that's the problem. If you receive some
of his traffic from upstreams of yours that don't accept his /24
advertisements, you will have to get them to him in some way. You will
simply need to find one route to him that works, and push all his traffic
out that way. What will stop this from working are:

	1) If the upstream just hands the traffic back to you because it doesn't
see your customer's /24s.

	2) If the upstream or something on the path does see your customer's /24s
and decides that you shouldn't be originating the traffic and so blocks it.

	It's theoretically possible that all of your upstreams will have one or
both of these problems, and hence you will have no way to get traffic to
your former customer.

	Worst case, your former customer can always get a few IP address from
another provider (router interface addresses are fine) and set up a tunnel
to you. That will allow you to get him his traffic if it winds up on your
doorstep. Unfortunately, many tunnelling techniques aren't truly
transparent, and may not support a payload MTU of 1500. :(

	Ugly, ugly, ugly.


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