shared-network question

Eric Helm helmwork at
Mon Oct 23 15:55:27 UTC 2006

Bruce Hudson wrote:
>> Its not desirable to have this on one DHCP server at this point. I was
>> thinking, that since there will be a giaddr in the DHCPDISCOVER I could
>> use the shared-network statement with both subnets I need, however leave
>> out the appropriate range on each DHCP server. What do you think?
>     How is your router supposed to know which giaddr to use when it sees an
> incoming DHCP broadcast? If you have something like a Cisco (you mentioned
> ip-helper), only the primary address is used in DHCP forwarding. If you put
> two ip-helper statements on your interface, both of your DHCP servers will
> see all requests. 
>     You need some other mechanism to decide which requests each server should
> handle.

Maybe I misunderstand the shared-network, but its my understanding that
with a shared-network, the DHCP server will hand out an IP from ANY
subnet in the shared-network, so if the giaddr is from 1 of the subnets
in the shared-network, shouldn't the server assign an address from
either of the subnets? If so, couldn't I just not include a range
statement for the subnet I don't want to assign addresses from?
Otherwise, if the shared-network doesn't work how I think it does, then
I agree with you, there needs to be some way to determine what subnets
to use since the giaddr will only be the primary address of the interface.

FYI, Cisco has a feature called 'ip dhcp smart-relay'.
If the ip dhcp smart-relay global configuration command is enabled, then
the server identifier override and subnet selection suboptions will use
the secondary IP address of the incoming interface when the same client
retransmits more than three DHCP DISCOVER packets.
Unfortunately, we are not using Cisco in this case...


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