For inter-site replication, one domain controller per site has the responsibility of evaluating the inter-site replication topology and creating Active Directory Replication Connection objects for appropriate bridgehead servers within its site. The domain controller in each site that owns this role is referred to as the Inter-Site Topology Generator (ISTG).
By Default the first Server has this role. If that server can no longer perform this role then the next server with the highest GUID takes over the role of ISTG.
