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DHCP failover not working

Posted on 2016-09-19
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Last Modified: 2016-10-19
I have to 2012 R2 server configured for DHCP Failover, and it was working fine.  Lately they haven't been communicating with each other.  If I set an IP address through the DHCP server only one sees it and the other just ignores it.  This leads to some confusion between the servers.  They are both Domain Controllers and DNS configured on both.  How can I force them to communicate with each other.  I tried doing the replicate failover scope option in the DHCP  and it works for the secondary one but the primary one tells me that they are already in sync.  
How do I get them to work together again?
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Question by:lcfrederickson
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by:Cliff Galiher
ID: 41805660
"If I set an IP address through the DHCP server "

Failover *only* syncs leases. It does not, and never has, done automatic syncing of reservations or scope options. Those must be set per server.
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by:lcfrederickson
ID: 41805737
The way I had it set up both DHCP servers shared the same scope and addresses.  When one went down the other would take over the full load.  It was working fine until the became disconnected somehow and aren't communicating anymore.
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by:Cliff Galiher
ID: 41805746
Scope? Yes.
Scope OPTIONS? No.

Address leases? Yes.
Reserved addresses? No.

DHCO failover in 2012/R2 *never* supported syncing the things I listed above. So if you think you remember it doing so, you are misremembering.
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by:Cliff Galiher
ID: 41805752
Official technet page:

https://technet.microsoft.com/en-us/library/dn338983(v=ws.11).aspx

Note the "important" box:

"If changes are made to a failover-enabled scope, you must manually replicate these changes to the partner server in order to synchronize scopes on both DHCP servers."

Changes must be MANUALLY replicated. Automatic replication of reservations and scope options DOES NOT OCCUR. This is official Microsoft documentation.
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by:lcfrederickson
ID: 41805762
OK so I set up both server with the same scope but as I added reservations they couldn't  communicate that?  If I have reservations am I better off not using the failover?  It sounded really good and like I said it seems to work for awhile.  The reservations were seen in both servers.
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by:lcfrederickson
ID: 41805764
What does the replicate failover scopes do?  Not sure I understand
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by:Cliff Galiher
ID: 41805773
A failover scope will share lease information. But scope options and reservations are not automatically replicated. Not sure how else to repeat that. What specific question do you have?
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by:lcfrederickson
ID: 41805785
I'm not trying to be a problem but I did ask 2 specific questions.  If I use reservations am I better off not using using the DHCP Failover?  Number 2, what does the replicate failover scopes options do if the scopes won't replicate?   I did set up the scopes on each server but if they can't share the reservations then do I just not do reservations?  That would be pretty difficult.
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by:Cliff Galiher
ID: 41805787
If you *only* use reservations, DHCP failover may be of limited value. That depends on if you also use standard dynamic leases.

Replicate failover scopes does a one-time point A to point B copy of settings. It doesn't combine them. It doesn't copy from point B back to point A. It does no conflict avoidance or resolution. It is a manual replication point in time.
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by:lcfrederickson
ID: 41805789
The vast majority of addresses and standard dynamic.  Most of the reservations are for NAS Backups and computers that I want to let through our SonicWall firewall.  If I can't use the reservations do I need to have 2 DHCP servers for the DHCP Failover and another one to do the reservations?  I'm not trying to be a pain, I just don't understand if I can't use reservations on the DHCP Failover servers what is the solution?
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by:lcfrederickson
ID: 41805790
Would I be better off just keeping a backup of the reservations and a backup DHCP ready in put them in in case of a failure of the DHCP server?  That would mean down time.
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Cliff Galiher earned 500 total points
ID: 41805796
You can certainly use DHCP failover. And if you use standard dynamic leases, it is probably worth it to do so. As to your reservations, you just have to get in the habit of making changes on both servers or always using one server to make edits as a "master" server and then forcing a manual replication whenever you make changes. It is a workflow issue,  not a technical one.
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by:lcfrederickson
ID: 41807341
Thanks!  Learned alot!
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