Explain loopback to me again, please

[Tom2018]

I can already create a new Network Element with the same IP...
And there is no warning about a duplicate

[Ronan]

But your 3300 doesn't seem to be part of a cluster. Mine are, and when trying to sync from one to another, I get an error telling me there is a duplicate IP address in Network Elements.

[Tom2018]

Yes, you're right.
In a cluster this seem don't working

[VinceWhirlwind]

I'm not sure what you're trying to do with creating dummy Cluster members.

To get a loopback working, you create a Trunk Group that refers to the same PBX you are on with a Local Profile Number that refers to a Trunk Service number that is *the same* as the Remote Profile number, which are in turn a Trunk Attribute number you've created for the purpose.
The forms are
ICP/XNET Trunk Group
ICP/XNET Trunk Profile
Trunk Attributes.
 
Just follow the Loopback instructions and it works.