Re: [dhcwg] Question: in RFC3046 why did Agent Subnet Mask Sub-optiondie

On Tue, Mar 06, 2007 at 09:35:08PM +0000, Simon Kelley wrote:
> The Agent Subnet mask would provide the needed information and make 
> configuration for remote networks work in the same way as for local 
> networks. This would make my life easier because my users would no 
> longer omit netmask information and wonder why their DHCP doesn't work.

Except that some DHCPv4 relays do not have addresses on the networks
they are relaying...so they don't have this knowledge.

Some don't set giaddr, but do provide the relay agent option.  So
a subsequent relay agent that does set giaddr would be disallowed
from placing their own relay agent option in the packet and
couldn't deliver the subnet mask if they had it...

Some do set giaddr (to an address on a different interface), and
therefore have to be configured with a subnet selection option
contents.  So far as I know, none of them expect 'CIDR notation'
for this configuration value, but I suppose it is at least
possible that they might be made to.

But I'm more curious about what you intend to do with multiple
subnets on one broadcast domain...which subnet should the relay
provide the mask for?

Are relays going to be required to know about all the subnets
attached to a physical interface?  If they're too dumb to have
an IP address, but smart enough to be able to supply useful
information in a relay agent option, are they going to need a
way for routers to inform them what subnets they should
advertise to the DHCP server?

I don't like the vector to this line of reasoning.

> To answer your question another way, I have a DHCP server which doesn't 
> need to be explicitly configured with the network topology, it derives 
> it. To make this work with relays need the relays to provide netmask 
> information.

I don't think this will work without creating some sort of
"subset of compatibility" (similar to the client-identifier
and domain-name options).

-- 
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