Splitting a block of Class C's

Andy andy at tigerteam.net
Fri May 25 00:39:31 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?

It really depends on what you *want* to do. If the /24's you have assigned
to them are non-contiguous then they will want to renumber anyway so they
can advertise an aggregate. If you want to help them out, tell them to go
ahead and advertise the /24's out of your aggregate. Some providers have
stricter filtering, but generally will be reachable through other
providers, so connectivity isn't a big deal. Sprint should be advertising
an aggregate of your IP space which will bring all traffic to their
network. Once it reaches Sprint's network, the more specific routes your
customer is advertising will go to them and the rest will come to you. I
can't see that this would add any additional load to your router.


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