Migrating DHCP servers
dhcp1 at thehobsons.co.uk
Mon Jan 10 10:03:47 UTC 2011
Peter Rathlev wrote:
>We're about to move several networks from being serviced by a Windows
>DHCP server to being served by ISC DHCP instead.
>What can we expect of trouble if we simply replace the "ip
>helper-address" configuration on the router? I tried poking around in
>the archives, and stumbled upon this:
>But it didn't make me significantly wiser.
>Two specific questions:
>1) Clients that already have a lease would keep asking (unicast) their
>current server. Will they do that for as long as the old server keeps
>acknowledging their lease, never returning to broadcast again to hear
>about the new server? If yes, do I have other options than either
>forcing all clients to reboot or disabling the old DHCP server?
Correct. As long as the old server keeps answering, then clients can
keep an address leased from it. Unless you take specific steps, then
nit's a bad idea to run multiple DHCP servers anyway - so you
wouldn't want to keep the old one active after the migration.
>2) With "ping-check true;", and assuming all clients will answer to ping
>requests, should I have any serious concerns about migrating without
>keeping lease state?
There are a lot of variables, and one that can cause problems is
dynamic DNS updates. If you were to just "switch off" an old server,
then you could find the new server unable to update the DNS as
records will already exist but they won't be "tagged" (with the
corresponding TXT record) as owned by the new DNS server.
With shortish leases and a fairly stable set of clients, you should
be able to just turn off one server and turn on the new one. As their
leases expire, clients will broadcast a request to use their current
address and the new server will acknowledge that - thus clients will
keep their current addresses.
Clients which come onto the network without a current address will
however get a new lease - and that could clash with an address
already in use. Ping check would detect that (subject to the "clients
respond to ping" you've already mentioned) - but will create an
Abandoned leases are only use in a "last resort" when all other
options have failed to provide a free lease - so you would probably
want to go through your leases and remove these once the migration
There is a trick you can use to your advantage. Bear in mind this is
only an incidental effect of the way internal tables are hashed, but
the ISC server current allocates unused addresses in a top-down (high
to low) order. So if your new pool extends to higher addresses than
teh old server had available, clients asking for a new address (ie
not requesting a specific address) will get one from the top of the
range where it doesn't overlap with the old one. This will
significantly reduce the likelyhood of a collision.
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