DHCP Failover and DHCP relay question

Simon Hobson dhcp at thehobsons.co.uk
Mon Mar 27 23:46:45 UTC 2006

Darren wrote:

>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).

Your setup WILL cause you BIG problems unless ...

>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.

You can have the modems and clients handled differently by the relay 
agent. Since you say this cannot be done then you are building 
yourself an ongoing headache.

>   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?

To work properly, both partners must get dhcp traffic, so the first 
thing you would need is to relay requests to all three servers. As it 
sounds like you can only specify two servers, this will be your first 
cause of problems.

Next you will need to configure each server to ignore clients that 
the others should be handling. I'm not familiar with cable modem 
setups so I don't know how easy/difficult that will be, but's it's 
going to be a complication. If you are using the standard Windows 
server then I do know that it has very minimal configuration 

ASAIK, the ISC server can still do bootp and failover, but not in the 
same pool. So provided you can split the clients by some means, then 
you can have a failover pool for clients, and a separate non-failover 
pool for bootp clients (modems).

There are, or at least have been in the past, a number of people on 
this list running cable modems - and I do know that they didn't have 
to resort to multiple servers to do it.

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