Migrating DHCP servers

Peter Rathlev peter at rathlev.dk
Mon Jan 10 21:31:08 UTC 2011

On Mon, 2011-01-10 at 10:03 +0000, Simon Hobson wrote:
> 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.

Yeah, I can see that. The "problem" is that the current DHCP server is
not our jurisdiction, to put it that way; we're Network Operations and
they're Server Management. But since disabling the old server doesn't
have to happen at some exact time I guess it's not that big a problem.

> 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.

That's definitely something we have to test. We'll start by migrating
the voice scopes (Cisco og Nortel telephony) and hope that won't give us
any problems. When all is good we'll start looking at the Windows Active
Domain clients.

> 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.

I was thinking we could start the new server and let it take any new
clients. After this we turn off the old server (or rather: disable the
now migrated scopes) and expect it to either NAK requests or not answer
them at all, thus forcing the clients into broadcast mode.

> 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 
> was complete.

Good point.

Thank you very much for the input, it's much appreciated. :-)


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