GAL sync and calendar sharing between Exchange 2 organizations?

Question

Microsoft Exchange

Colleagues, hello.
There are 2 organizations. Each has its own Active Directory, Exchange (2013 and 2016), and other infrastructure.
Initially, the firms were completely separate from each other, but now there is some kind of integration, connections between employees of departments are getting closer, and it is required that employees of one firm can easily find the mail of an employee from another organization. In addition, there was a move to our building and meeting rooms are now also used by both organizations.
In this regard, it is required
a) Set up GAL synchronization
b) Add the ability to view the calendar and book meeting rooms by another organization.

A two-way transitive trust relationship is configured between the organization domains. The network addressing is common, after the move we use one DHCP server for two.

So far, on the Internet, I have only found about GAL synchronization with PoSH scripts, and a way to share calendars for booking conference rooms by setting up federation trust through the Microsoft 365 admin center (and I don’t really want to go into cloud services).
Is there really no built-in ways to synchronize between trusted domains in such a powerful tool as Exchange?

Answer 1

There is a way to do this without Federation Sharing, using the Availability service for cross-forest topology.
How to configure -
If you configure according to the article above, you need to take into account that -UseServiceAccount has not been working since April. How to fix -
It didn't turn out as easy as in the article, I had to deal with a bunch of errors, but in the end everything works as expected.

Answer 2

You need to set up the so-called. federation sharing (see here ) or here