Dynamic vs static IP allocation w/r known MAC with 2 servers

Bruce Hudson Bruce.Hudson at Dal.Ca
Thu Apr 26 12:31:40 UTC 2007

>   Is it possible that two DHCP servers (ISC) live together in the same
>   VLAN, where the first always offer an IP address and the second no,
>   but is prioritary when it answers?

    Not really, at least not on the server side. DHCP is designed to be
client centric. The basic theory of operation is that the client selects
somehow between the various offers and chooses the "best" one. Normally
the best one is the first one is gets.

    We run two DHCP servers on the same VLAN here for historical reasons.
The key we found is to make sure only one answers any given query. Neither
server is authoritative. B is NAKing the requests because you configured
it to be authoritative. You've also told it that your two subnets are on
separate VLANs. If you try wrapping them in a "shared-network" block so
that it at least knows the subnet it is not authoritative for is on the
same VLAN as the one it is, this MAY work. I have not tried mixing this

    BTW, to do both on one server (not what you asked for I know), we use
something like:

	shared-network VLAN1 {
	    subnet  netmask {
		<subnet options>

                pool {		# KNOWN CLIENTS, from other VLANs
	            allow known clients;
	    subnet netmask {
		<subnet options>
	        pool {          #UNKNOWN CLIENTS
	            deny known clients;
Bruce A. Hudson				| Bruce.Hudson at Dal.CA
UCIS, Networks and Systems		|
Dalhousie University			|
Halifax, Nova Scotia, Canada		| (902) 494-3405

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