dhcp with two relay agents

fadey fadey at scancom.es
Thu Oct 30 19:41:09 UTC 2008


Great to hear it'll work. Thanks to everyone who followed the topic.

Regarding the CMTS tweaking. We are using cisco ubr7100/ubr7200. It is
possible to configure

cable dhcp-giaddr policy

This command will set primary IP address of the cable interface as a
giaddr in case of DHCPDISCOVER comming from cable modem. If DHCPDISCOVER
is comming from a host or emta it'll set the giaddr field to first
secondary IP of the cable interface. Cisco (unlike Arris) can't AFAIK
differentiate an emta from a host when setting giaddr field. Please,
correct me if I'm wrong.

Thanks once again and best regards.
> fadey wrote:
> >Thanks. It starts getting more clear. Basically now my question narrows
> >down to following:
> >
> >I have this setup:
> >
> >shared-network router1 {
> >   subnet router1.cable_modems {deny unknown-clients; ...}
> >   subnet router1.emtas {deny unknown-clients; ...}
> >   subnet router1.hosts1 {...}
> >   subnet router1.hosts2 {...}
> >}
> >
> >shared-network router2 {
> >   subnet router2.cable_modems {deny unknown-clients; ...}
> >   subnet router2.emtas {deny unknown-clients; ...}
> >   subnet router2.hosts1 {...}
> >}
> >
> >All router1 subnets share the same physical network. The same is true
> >for router2 subnets.
> >
> >Now there is a DHCPDISCOVER relayed from router2 with GIADDR set to
> >router2.cable_modems IP. It is an unknown-client (mac is not explicitly
> >specified in dhcpd.conf). In this case will dhcpd be "smart enough" to
> >DHCPOFFER an IP from route2.hosts1 subnet and NOT from router1.hosts*
> >subnets?
> 
> Yes, this differentiation based on GIAddr is automagic.
> 
> However, and this is something that I hope some of the cable modem 
> guys will chip with ...
> I'm not sure you need the shared network at all. It could be that the 
> cable equipment is capable of differentiating between the 
> infrastructure kit, user end cable modems, and user kit - in which 
> case I suspect the GIAddr received will be set to that of the 
> appropriate interface.
> Either that, or yu keep the shared network and can use something like 
> vendor class to split the equipment into the appropriate subnets.
> I've only vaguely followed the cable modem threads, so hopefully 
> someone who's actually used this stuff can chirp up and fill in the 
> blanks.
> 



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