shared-network question

Gary Nielson gary at
Mon Oct 23 17:02:34 UTC 2006

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-----Original Message-----
From: dhcp-users-bounce at [mailto:dhcp-users-bounce at] On Behalf
Of Eric Helm
Sent: Monday, October 23, 2006 11:55 AM
To: dhcp-users at
Subject: Re: shared-network question

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
>     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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