Migrating from a Cisco Routers DHCP to ISC DHCP

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Oct 19 12:40:58 UTC 2009

Zemke, Kai wrote:

>One thing I'm thinking of is the question: is there a way to keep 
>the old ip dhcp binding table from the cisco router? And maybe copy 
>it to the isc dhcp server?
>Or what ways are pefered to make all clients get a new ip address on 
>the new server. So that there are no adresses on the net twice 
>because the lease hasn't expired yet
>but the server offered this ip to a new client because he just 
>wasn't aware of an old lease.

There are several ways to do this, what will work for you depends on 
your network and clients.

One technique would be to shorten the leases on the old server - 
working progressively in stages until you are down to a few minutes. 
Then at a quiet time turn off the Cisco server, wait for the 
configured lease time, and turn on the new server. All clients will 
see a network outage as their leases expire, but will then renew with 
the new server. For most clients, they will get their old address 
back - they'll request it from the server and since it should be free 
then that address will get assigned by the new server.

Another technique would be progressively reduce the size of the pool 
(IIRC add to the excluded address range(s)) on the Cisco server, wait 
for the leases to expire, and then add those addresses to the new 
server. Any clients with addresses in the bit of pool that you chop 
off will be forced to get new leases - either different addresses 
from the old server, or from the new server. Over time you can force 
all the clients over to the new server. You can speed things up by 
cutting down lease times on the old server in advance, and minimise 
impact by timing the alterations so that leases will expire during 
quiet times.

Simon Hobson

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