If you are planning a DHCP migration to an on-premises Windows Server 2025, this post will be of assistance. Having migrated numerous DHCP servers over the years, I have encountered various issues. In this blog, I will discuss common problems that may arise during a DHCP migration, symptoms, causes, and practical steps to mitigate each. By the conclusion, you will have a comprehensive checklist to ensure your DHCP migration to Windows Server 2025 proceeds smoothly.

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Scope Migration Challenges

Symptom: After moving DHCP to the new server, some clients are not receiving IP addresses or are receiving incorrect configurations. Missing scopes on the new server or clients on specific subnets failing to renew their leases may be observed.

Cause: Migrating DHCP scopes involves more than merely copying settings. Issues can occur if not all scopes or options were correctly exported/imported, the new server is not reachable by all subnets, or the server’s identity (IP or hostname) has changed. For instance, DHCP clients send renewal requests to the old server’s IP address; if your new server uses a different IP, those initial renewals will fail. Clients will eventually broadcast to locate the new server, but that transient failure may cause delays. Additionally, network devices must be updated: if routers or relay agents (IP helpers) still point to the old DHCP server, clients on those VLANs will not reach the new one. Mistakes in migrating many scopes with custom options or reservations or using outdated tools can lead to missing configurations on the new server.

Resolution:

  • Utilize PowerShell cmdlets or the DHCP console backup to export your scopes and leases from the old server and import them to the new server. This ensures that all options and reservations transfer accurately.
  • If feasible, during the transition, assign the new DHCP server the same IP address (and name) as the old server.
  • Reconfigure routers or switches relaying DHCP requests to point to your new Server 2025’s IP.
  • Verify that every scope, IP range, exclusion, and option (DNS servers, gateway, etc.) aligns with the old server.
  • Ensure the new DHCP server’s network interfaces are configured correctly and listen to the appropriate NICs.

DHCP Database Corruption or Incompatibility

Symptom: The DHCP service on the new server refuses to start, or it starts but logs Jet database errors or exhibits missing leases or scopes.

Cause: The DHCP database (Dhcp.mdb) might be corrupted or incompatible with the new server. Corruption can occur on the old server, while incompatibility may result from directly copying the database instead of using supported migration tools.

Resolution:

  • Utilize the DHCP console’s Backup feature on the old server and restore on the new server.
  • Ensure the new server’s DHCP service points to the correct database location and that permissions are set correctly.
  • Attempt a repair using Jetpack or esentutl if corruption is suspected.
  • Monitor DHCP Server event logs for errors.
  • Confirm sufficient disk space is available on the server for DHCP operations.

Failover Configuration Issues

Symptom: Failover replication is not working, leases are unsynchronized, or clients receive duplicate IPs.

Cause: Misconfigured or outdated failover relationships, time skew, or port blocks can cause issues. Changing DHCP server IPs without updating failover settings can disrupt the relationship.

Resolution:

  • Carefully plan a staged migration of failover partners.
  • Remove and recreate failover relationships if IPs change.
  • Open TCP port 647 between servers and ensure time synchronization.
  • Verify failover health in the DHCP console.
  • Ensure both servers have matching scope configurations.

DHCP Server Authorization Issues in Active Directory

Symptom: DHCP service fails to start or stops shortly after starting; clients do not receive IP addresses.

Cause: DHCP servers in an AD environment must be authorized in the Active Directory. If not, the service will not operate.

Resolution:

  • Authorize the new DHCP server using the DHCP MMC or PowerShell.
  • De-authorize and remove the old server after migration.
  • Clean up old DHCP server entries in AD Sites and Services.
  • Ensure network connectivity and permissions for AD access.

IPv6 Configuration Complexities

Symptom: IPv6 clients do not receive addresses or proper DNS settings; DHCPv6 scopes are missing or non-functional.

Cause: Misconfiguration of DHCPv6 scopes, absent static IPv6 address on server NICs, or reliance on SLAAC instead of DHCPv6.

Resolution:

  • Determine if DHCPv6 is necessary and if scopes were previously utilized.
  • Assign static IPv6 addresses to server NICs.
  • Create or import DHCPv6 scopes with accurate options.
  • Configure routers to support DHCPv6 (RA flags).
  • Test client renewals and validate DNS updates.

DNS Dynamic Update Conflicts Post-Migration

Symptom: Following migration, DNS entries are missing or incorrect. Some records may disappear, or duplicates might appear.

Cause: Improper cleanup behavior on the old server, lack of proper DNS update permissions on the new server, or duplicate record ownership.

Resolution:

  • Decommission or disable DHCP on the old server immediately after cutover.
  • Disable “Discard A and PTR records when lease is deleted” before removing old scopes.
  • Set correct DNS update credentials on the new server.
  • Re-register affected clients or use ipconfig /registerdns.

Missing or Misconfigured Reservations

Symptom: Devices with reservations do not get expected IPs or receive the wrong devices.

Cause: Missing reservations in the import, incorrect MAC addresses, or inactive reservations.

Resolution:

  • Confirm that all reservations are present and properly configured.
  • Test each reserved client to ensure the correct IPs are issued.
  • Remove duplicate or unused reservation entries.
  • In failover configurations, ensure reservations are synchronized between partners.

Service Startup Failures or Role Installation Glitches

Symptom: DHCP service does not start, crashes, or errors occur during role installation.

Cause: Missing authorization, corrupted or inaccessible database, port conflicts, or incomplete installation.

Resolution:

  • Verify that the server is authorized in AD.
  • Reinstall the DHCP role if required.
  • Ensure database paths and permissions are correct.
  • Confirm static IP configuration on the DHCP server.
  • Check Event Logs for specific service errors.

Compatibility with Legacy DHCP Clients and Relay Agents

Symptom: Legacy devices (phones, printers, embedded systems) do not receive IPs or required options.

Cause: Legacy clients may require specific DHCP options or broadcast replies or have compatibility issues with modern configurations.

Resolution:

  • Enable broadcast replies if necessary.
  • Verify all DHCP options (e.g., 66/67 for PXE, 150 for VoIP).
  • Test using known compatible clients.
  • Update relay agents (IP helper configurations).
  • Ensure that no MAC filtering blocks legacy device addresses.

Conclusion

Migrating DHCP in an on-premises environment to Windows Server 2025 involves numerous critical steps. With diligent planning, thorough documentation, and careful validation, most common issues can be avoided or promptly addressed. Conduct meticulous testing, decommission old servers carefully, and validate each configuration element post-migration.

We wish you success with your DHCP migration. If you have any questions or experiences to share, please do not hesitate to contact us.

Thanks,

Cristal