Case:– Problem while migrating a DHCP Server from 22.214.171.124 to 126.96.36.199 for segment 10.1.1.0/24 users. Static and Dynamic routing is not needed since only one router is involved. Connectivity from end to end is up and working for the devices shown in diagram.
Both of the DHCP servers are having identical scopes. Currently LAN devices of 10.1.1.0/24 are using OLD DHCP server for dynamic IPs. Before the DHCP migration, scope is configured with Lease of 8 days so all users acquired IPs using 8 days lease. We decided to change the lease to 4 hours from 8 days on DHCP for scope 10.1.1.0/24 on the old server and moved the DHCP server IP using “ ip helper-address” under Interface Gateway 10.1.1.1 e0/0 on Router R1 after 5 hours of changing the lease time . We copied all of the DHCP client lists from the Scope to new DHCP Server using script during the change window. Most of the users went home and restarted their machines next day- they were all fine. But those who did not restart the machine, they got IP conflict after few hours.
Please provide the reason for “IP Conflict”. Also, what went wrong and what was the thing we missed for smooth transition?
Resolution :- The users who did not restart workstations– always had old lease time supplied from Old DHCP Server 188.8.131.52 . Even though we changed the time of lease from 8 days to 4 hours on new server ( 184.108.40.206)- but DHCP does not push these changes to clients. So, once the DHCP server is migrated, That client( with 8 days lease) will still reach out to the old DHCP Server for renewing its lease time because we did not remove Old DHCP server from network – which is a mistake. Also that workstation will still have IP 220.127.116.11 against “DHCP Server” in “IP Configuration”, therefore, unicast will happen to Old DHCP. So shut the old DHCP server or disable its relevant scope.
If we do not shut or disable the scope- then the same IP will be allocated to some client which will create problem of ” IP Conflict” in LAN environment.