split scoping

[Niall O'Reilly]

Chris Buxton wrote:
> The usual way to set up split scopes in the Microsoft world is to use one subnet,
> with disjoint address pools within (or the same address pool but half excluded,
> which is functionally the same thing).
> 
> With ISC DHCP, this has a problem: If the servers both consider themselves
> authoritative, they'll NAK each other's leases when they come up for renewal.

	They won't.

	We've been doing this for ages, for dozens of LANs.
	Back whenever, I formed the opinion from reading this list
	that failover was going to be a heap of trouble.  Once I had
	this set up, and "just working", there never seemed to be a
	good reason to fix it to use failover.
	
	For example, one server's configuration has this:

         subnet 137.43.162.0 netmask 255.255.255.0 {
             option subnet-mask 255.255.255.0;
             option routers 137.43.162.1;

             pool {
                 range 137.43.162.129 137.43.162.190;
                 deny unknown clients;
                 max-lease-time 7200;
             }
         }

	and the other's has a complementary configuration:

         subnet 137.43.162.0 netmask 255.255.255.0 {
             option subnet-mask 255.255.255.0;
             option routers 137.43.162.1;

             pool {
                 range 137.43.162.193 137.43.162.254;
                 deny unknown clients;
                 max-lease-time 7200;
             }
         }

	IIRC, NAK's are only given if the address is invalid with
	respect to the base address and network mask for the subnet.

> But if you use different subnets, rather than splitting the subnet, then things
> like zeroconf won't work.
> 
> I'm sure someone will correct me if I'm mistaken.

	Gladly!  8-)

	Best regards,

	Niall O'Reilly
	University College Dublin IT Services