Best practice on removing IP Scopes(subnets)
dhcp1 at thehobsons.co.uk
Fri Apr 24 15:22:35 UTC 2009
Jeremy Phillips wrote:
> We are currently in the process of moving our IP space. I'm
>trying to remove the old IP subnets from the dhcp server without
>Customers or having our helpdesk have 1000's of calls from customers
>that need to get a new IP. Essentially we will be moving 20+ class
>I was just wondering how some of you went about this very procedure
>without effecting service.
<pedant>I believe the word you are after is affect, not effect. I
believe you wish to continue effecting service.</pedant>
How you go about the change depends on how urgent it is, and what
your equipment capabilities are.
The most graceful way is to simply stop handing out addresses*,
configure the routers with shared subnets (new+old on same
interface), and wait. Over time the clients will migrate to the new
subnet as their leases expire. Many clients will get a new address at
boot time (or more technically correct, as they bring up an interface
after shutdown or sleep or move) and interruption to connections will
* One way is to leave the pool declarations in place and add "ignore
booting" to them. Another is to simply remove the pool. If you remove
the subnet declaration then clients will get a NACK, so they may
switch address mid session and dropped connections are more likely -
and see below for DNS implications.
With advanced planning, you can speed things up by reducing the lease
time in advance - eg if you have 4 week leases, then reduce this in
stages to perhaps only a day (or less) over the month or so prior to
the changeover. That way, when you stop renewing leases for the old
addresses, it won't take long for them to expire.
If you can't (or don't want to) do parallel running at all, then you
will want to get clients onto really short leases. Reduce lease times
in advance - eg gradually cut them down
4wk-2wk-1wk-4day-2day-1day-12hr-3hr-... Ie, at t-4weeks, cut the
lease time down to 2 weeks, at t-2weeks, cut the lease time down to
1week and so on. Just remember that the shorter you get the lease
times, the quicker your customers will notice if your server breaks !
At the appointed time, change configs - router interface addresses
and DHCP server. Clients will lose connectivity until they lease a
new address, but if your lease times are short, then this should be
too much of a problem if you time it right AND inform customers
accordingly. In a corporate environment I'd plan to do this when the
lease number of people are on the systems - which will obviously
depend on your shift patterns and countries served.
There will be problems with dynamic DNS entries - if you just delete
subnet definitions then I've an idea that the server will delete the
lease records, but might not delete the corresponding DNS entries. If
you do a graceful changeover, then DNS entries will get deleted as
leases expire, and new entries can then be created on the next client
Sadly, the last time I had to do this, I was still using the broken
SCO DHCP server - only does a NACK if it's the first query since
server startup ! Had to walk round the office doing manual
release/renew on all the PCs :-(
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