F5 clustering tips

Intro
I was having trouble with one of my new F5 clusters. I thought I’d be clever and add two new F5’s to the existing cluster, and then remove the old members when everything looked good.

But the device groups somehow remembered the old servers, even though they weren’t in the device groups. Eventually I could not even sync the two new servers together.

Here’s a concise summary of the steps which worked, taken from this article:

Hi Manuel, do you try to setup a sync-failover device-group containing three units? To establish device trust I would recommend to force two units into offline state. Remove the machines from the existing sync-failover device-group (repeat on each machine if required) and delete the sync-failover device-group. Now reset the device trust on all machines. Next step will be to use the active machine to add both offline machines as peers. Now all three units should show up in each machine´s device list. On the active unit create a new device-group of type sync-failover with network failover enabled. Add all machines to the new device-group. This will be synced to all machines and you can release them from forced offline. Time for the initial sync now. Thanks, Stephan

I get a little confused about sync-only vs sync-failover. That difference is clearly discussed there as well:

“Sync-only” will not allow to synchronize LTM configurations.
To synchronize an LTM config all units need to belong to the same “sync-failover” device-group.

I had afraid and ignorant of the need to reset device trust. For good measure I deleted all device groups. Then this procedure seems to leave me with an auto-created datasync-global-dg (sync-only). Since there was no sync-failover group I manually created one I called sync-LTM-configs. Another auto-created, sync-only group is the device_trust_group. So I see three device groups in all when I am on the sync page, but only two when I am on the device group page.

Conclusion
I had afraid and ignorant of the need to reset device trust in cleaning up an F5 cluster with old and new members. But once I did that and followed the recipe above, I was good to go.

References and related
The discussion on the F5 Devcentral site: https://devcentral.f5.com/questions/f5-sync-options

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