Splitting a block of Class C's

Steve Gibbard scg at gibbard.org
Fri May 25 02:14:25 UTC 2001

On Thu, 24 May 2001, Albert Meyer wrote:

> One of my ISP clients is leaving, and the person who was assigning IP's 
> when they became our client chose to give them class C's scattered 
> throughout our block. These are Sprint IP's which are assigned to us with 
> our Sprint circuit. Our former client is getting a Sprint T1 and asking 
> Sprint to route these class C's to them rather than us, and they tell me 
> that Sprint appears willing. They don't have an AS, but they're getting 
> another local ISP to advertise the IP's for them. I normally wouldn't agree 
> to participate in such a mess, but we're shutting off the Sprint circuit in 
> a couple of months, and I can't see making them re-IP hundreds of domains 
> under the circumstances. Has anyone done something like this? I'm wondering 
> how much it will increase the CPU load on my router. I'm already running at 
> 20% average, and if it gets much over that I start dropping ICMP packets. I 
> know enough BGP to stop advertising the appropriate class C's, but I'm not 
> sure that this won't cause problems that I haven't considered. Will anyone 
> refuse to accept advertisements which send adjacent /24's to different 
> places? Is this an officially "broken" setup, or is it just ugly?

Just ugly, and as others here will no doubt point out, unfriendly to the
size of Internet routing tables everywhere.

If you really want to do this, you should keep anouncing your whole IP
address block, since the /24s will get filtered in some other places.
Your former customer (if they really want to do this) should be anouncing
the /24s, which will be more specific and will thus send traffic bound for
their space to them.  You will need to open your filters to let those
announcements in, or else you won't be able to communicate with your
former customer.

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.

That said, just because you can do something doesn't mean you should.  
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.  Growing the size
of the Internet routing table, or at least the routing table as seen by
those who don't filter out /24s, is a good thing to avoid when you can
easily do so.  IP renumbering is a big pain, but it's unfortunately a
normal part of switching upstream providers for those without portable IP

Are you big enough to get a portable IP address block from ARIN?  Since
you say you're dropping your Sprint connection in a few months, you will
presumably have to renumber then.  If you want to be really nice, you
could bite the bullet and do it now, turning over the old block to your
former customer.  Alternatively, it's common practice to tell your
customer that they have to renumber out of your space when they stop
buying connectivity from you, and from your perspective that's probably
the easiest way to handle the situation.


Steve Gibbard				scg at gibbard.org	

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