Hi,
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To get trusted forest on IPAM server, please make sure that the managed account must be a member of the Domain Admin group in the trusted forest.
I found an article regarding of "Manage Resources in Multiple Active Directory Forests" for your reference.
https://video2.skills-academy.com/en-us/windows-server/networking/technologies/ipam/manage-resources-in-multiple-active-directory-forests
Regarding the issue DHCP server cannot be discovered.
- Please make sure that DHCP server role is not installed on the IPAM server. If the DHCP server role is installed on the same server with IPAM, DHCP servers will not be discovered on the network.
- IPAM discovers DHCP servers that are authorized in the Active Directory domains you specify and that respond to a DHCPInform message.
Please make sure that the DHCP server has been authorized in the AD and it can respond to a DHCPInform message. - Please help to verify that at least one IPv4 scope is configured on a DHCP server, and that the IPAM server has a TCP/IP connection to the DHCP server.
Here is an overview of IPAM guidance for your reference:
Getting Started with IPAM
Hope my answer will help you. Thanks!
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Best Regards,
Sunny