DHCP Relay and Subnet Selection question

Robert Fisher rfisher at sitestar.net
Tue Mar 28 20:07:24 UTC 2006

	I'm dealing with a cable modem plant and soon will run
into the issue that the /22 subnet I presently have for the CPE
devices will be exhausted.

	Presently, I'm just using a configuration on the CMTS that
allows you to specify on the cable interface both a primary and
secondary (or list of secondary) IP addresses -- The primary is
submitted as the giaddr for the CM network while the first secondary
address is submitted for the giaddr of the CPE network.

	Thankfully this makes the dhcpd configuration very simple, as
all I had to do was specify subnet clauses with a deny unknown-clients
as well as boot-unknown-clients off, along with a separate group clause
in which I list host entries for all valid cable modem clients.
	Perhaps I could get by with just one of these, but I wasn't clear on 
the distinction when this went up and since I haven't seen anything 
obviously broken :-)

	Currently, the only thing I have in the subnet definition for
the CPE network is just:

subnet	X.Y.Z.0 {
	option routers		X.Y.Z.1;
	option subnet-mask;
	range			X.Y.Z.2 X.Y.(Z+3).253;

	So, my question is -- how can I handle multiple subnets for the CPE 
group when the giaddr will always be X.Y.Z.1 -- At this point, I'm
not particular about which CPEs are assigned an IP Address from the
X.Y.Z.0/22 network or say from an A.B.C.0/24 or whatever network as I've
not even been tracking CPE MACs.
	Is this the sort of situation a shared-network clause would handle and 
why?  My apologies for asking something I've seen in the
list archives before, but this isn't something I really understand and
unfortunately the only 'test' environment I have for this is the live
production area for the clients.  :(

Thanks in advance,
Robert Fisher

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