The CM is just a bridge for that traffic. It has a management IP assigned to it by the provider but that's a different network so to speak.
Phil -----Original Message----- From: "Jay Ashworth" <j...@baylink.com> Sent: 12/29/2014 12:52 PM To: "NANOG" <nanog@nanog.org> Subject: Re: Charter ARP Leak ----- Original Message ----- > From: "Brett Frankenberger" <r...@rbfnet.com> > On Mon, Dec 29, 2014 at 12:27:04PM -0500, Jay Ashworth wrote: > > > > > > Valdis, you are correct. What your seeing is caused by multiple IP > > > blocks being assigned to the same CMTS interface. > > > > Am I incorrect, though, in believing that ARP packets should only be > > visible > > within a broadcast domain, > > broadcast domain != subnet Yeah; I didn't use the right term. That's why my networks are small. :-) > > and that because of that, they should not be > > being passed through a cablemodem attached to such a CMTS interface > > unless > > they're within the IP network in which that interface lives (which > > is > > probably not 0/0)? > > > > This sounds like a firmware bug in either the CMTS or the > > cablemodem. > > int ethernet 0/0 > ip address 10.0.0.1 255.255.0.0 > ip address 11.0.0.1 255.255.0.0 secondary > ip address 12.0.0.1 255.255.0.0 secondary > > The broadcast domain will have ARP broadcasts for all three subnets. > > Doing it over a CMTS doesn't change that. Ok. But the interface to which the cablemodem is attached, in the general single-DHCP-IP case, is a /24, is it not? The example Valdis posted had 5 or 6 different /24s from all over the v4 address space; that seems exceptionally sloppy routing... I have seen ARP-traffic-not-for-me come through a cablemodem in the past as well, but it was *uniformly* for the /24 in which my modem's address lived that day. Cheers, -- jra -- Jay R. Ashworth Baylink j...@baylink.com Designer The Things I Think RFC 2100 Ashworth & Associates http://www.bcp38.info 2000 Land Rover DII St Petersburg FL USA BCP38: Ask For It By Name! +1 727 647 1274