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AZLearner-5762 asked SunnyQi-MSFT commented

How to restore out-of-sync DHCP Servers in HA?

I have a pair of Windows Server 2012 R2 DHCP servers in HA that suffers the infamous error "duplicates A BINDING-ACK message with transaction id: xxxxxx was received for IP address: x.x.x.x with reject reason: (Fatal conflict exists: address in use by other client ) from partner server:" I ran Invoke-DhcpServerv4FailoverReplication but it still produces many BINDINK-ACK errors. So I stopped the second DHCP server in order for clients to obtain stable IP leases. Now I am planning on bringing the HA back. My understanding is I can:

  1. bring the second DHCP server online

  2. Remove the failover relationship

  3. Re-establish the failover relationship

Hopefully it can sync the servers again. However, I am curious what happens after I've removed the failover relationship. Will both servers keep their own set of active leases (the second DHCP server apparently has invalid leases since it was offline)? It seems I need to manually remove all the scopes from the second DHCP server that was offline before.

If I re-create the failover relationship from the first DHCP server shortly after the removal of relationship, will the second DHCP server sync up with the first DHCP server's active leases? How does it know whether to sync from server A to server B or vice versa if I don't remove the scopes on the second DHCP server?

Thanks.

windows-dhcp-dns
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Hi,

Before we go further, I would like to know that's the failover type between these 2 DHCP servers? Hot Standby or Load Balance?

Best Regards,
Sunny

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Hi Sunny,
They are in Load Balance mode.

Thanks.

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Hi,

Just checking in to see if the information provided was helpful.

If yes, you may accept useful reply as answer, if not, welcome to feedback.

Best Regards,
Sunny

0 Votes 0 ·

Hi,

Just checking in to see if the information provided was helpful.

If yes, you may accept useful reply as answer, if not, welcome to feedback.

Best Regards,
Sunny

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1 Answer

SunnyQi-MSFT avatar image
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SunnyQi-MSFT answered

Hi,

Many thanks for your update.

For DHCP failover servers, if the one of DHCP failover server becomes unavailable, the partner server will offer a temporary lease to the client for the duration of the MCLT at 7/8 of the scope lease time. Once the DHCP failover server becomes available again, it will begin communications by sending an update request to the partner server. When communications are fully reestablished, at 7/8 of the MCLT lease time, the DHCP failover server will offer a lease to the client.

Will both servers keep their own set of active leases (the second DHCP server apparently has invalid leases since it was offline)?

I did a test in my lab and the answer is no. It depends on which server is the primary server (the server you configured failover relationship), if the partner server didn't own these scope before the failover relationship establishment, the active leases in all the scopes will be removed from the partner server.

If I re-create the failover relationship from the first DHCP server shortly after the removal of relationship, will the second DHCP server sync up with the first DHCP server's active leases?

Yes. I have tested in my lab. After deleting DHCP failover relationship, all the scope will be removed from the partner server. And if I re-created the failover relationship, all scopes on the active server will be synced to the partner server again.

How does it know whether to sync from server A to server B or vice versa if I don't remove the scopes on the second DHCP server?

Both DHCP servers in a failover relationship must maintain a persistent TCP connection with each other. DHCP failover partners establish and maintain this connection on port 647, and use it to exchange operational state information and lease information.

For more details, please refer to the following link:

DHCP Failover Communications

Best Regards,
Sunny


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