DHCP Failover and DHCP relay question

Darren perl-list at network1.net
Mon Mar 27 21:24:29 UTC 2006

Mr. Hankins (or anyone who knows the answer to this question),

v.3.0.3 of ISC DHCP

Lets say we have DHCP failover partners at and 
and a cable modem bootp server running on windows at for 
example.  The client DHCP of a cable modem network will be going to the 
DHCP failover partners.  The cable modem bootp traffic will be going to 
the windows bootp server.  It is not possible in this example to place 
the bootp on the failover partners (actually, I don't think the ISC DHCP 
server supports bootp on failover anyways).

We have an old USR cable access router that will be acting as the DHCP 
relay agent for both cable modems and the clients behind them.  While 
this router does support failover, it does not support separation of the 
client/cable modem traffic.  Therefore, to make this work we would set 
the primary DHCP server on the cable access router to so 
that the cable modem bootp traffic goes to the windows server.  We will 
set the secondary DHCP server to so that the client DHCP 
goes to the first failover server.

Can anyone tell me if this will work?  Specifically, what will happen if 
the hash check on the client MAC Address says that the secondary DHCP 
server should answer, but the secondary server never received the DHCP 
Discover message from the client?  Will the primary client answer?  If 
so, what parameters will determine if it answers (IE, what should these 
options be set too:

        max-response-delay 5;
        max-unacked-updates 5;
        mclt 600;
        split 128;
        load balance max seconds 5;

This failover cluster will also be used by other DHCP Relay agents that 
do indeed support failover properly, and many of which are not cable 
modem networks at all.

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