What is the ISTG? Who has that role by default?

Windows 2000 Domain controllers each create Active Directory Replication connection objects representing inbound replication from intra-site replication partners. 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).