[olug] VNC w/Qwest

[Obi-Wan]

>> When two DHCP servers both operate in the same IP space, they must
>> know about each other and their their database.  This functionality
>> is built into the ISC DHCP server to handle redundant pairs.
> 
> According to the RFCs a client initiates the DHCP request by
> broadcasting a DHCPDISCOVER message...

Yes, that's how it work.

> It's been awhile since I really had to study the RFCs (I worked with a
> team of developers that created their own LDAP enabled DHCP server a
> few years back), but I don't recall the RFCs specifying that each
> server share a common persistent storage, but obviously that would be
> one way of doing it.

DHCP failover is actually an IETF draft, but it hasn't changed much in
recent years and has been implemented in ISC's DHCP server for some time.
   http://www.ietf.org/proceedings/04mar/I-D/draft-ietf-dhc-failover-12.txt
It specifies how a redundant DHCP server pair should work.  They only
need a shared storage if they're handing out IP's in the same IP space.

> Another possibility would be to allocate
> different ranges to different servers and configure them so that they
> only respond to DISCOVERs from certain LAN segments.

Yes, that would work, but then what's the point of having multiple servers?
You get some redundancy, but at the cost of cutting your usable IP space
in half.

> Some vendors (Cisco
> for sure) have features in at least some of their switches that will
> prevent DISCOVER messages from being broadcasted, the switches
> recognize DHCP broadcasts messages when they hit the switch and rather
> than sending them too all the hosts on the switch, they can relay
> those directly to the DHCP server.

That's not a bad idea.  It does blur the lines between switches and
firewalls.  I know that most switches (at least the enterprise-class
ones) will handle DHCP forwarding, so that one DHCP server can manage
many different subnets without requiring that new clients be able
to broadcast DHCPDISCOVERs to the entire network.

-- 
Ben "Obi-Wan" Hollingsworth                             obiwan at jedi.com
   The stuff of earth competes for the allegiance I owe only to the
     Giver of all good things, so if I stand, let me stand on the
       promise that You will pull me through.  -- Rich Mullins