18. What is the KCC?

Windows admin interview questions (includes Vista)

With in a Site, a Windows server 2003 service known as the KCC automatically generates a topology for replication among the domain controllers in the domain using a ring structure.Th Kcc is a built in process that runs on all domain controllers.

The KCC analyzes the replication topology within a site every 15 minute to ensure that it still works. If you add or remove a domain controller from the network or a site, the KCC reconfigures the topology to relect the change.

KCC is Knowledge Consistency Checker, which creates the connection object that links the DCs into common replication topology and dictates the replication routes between one DC to another in Active Directory forest.

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

Windows admin interview questions (includes Vista)

Intersite Topology Generator (ISTG), which is responsible for the connections among the sites. By default Windows 2003 Forest level functionality has this role. 
By Default the first Server has this role. If that server can no longer preform this role then the next server with the highest GUID then takes over the role of ISTG.

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).

17. What’s the difference between a site link’s schedule and interval?

Windows admin interview questions (includes Vista)

Any time two networks are separated by links that are heavily used during parts of the day and are idle during other parts of the day, put those networks into separate sites. You can use the ability to schedule replication between sites to prevent replication traffic from competing with other traffic during high usage hours.
In simple words you can define it as the time when you allow the replication to happen.
Interval is also a part of schedule but it takes cares of the replication polling frequency. In other words in a said schedule of say 9:00 AM to 1 PM replication polling shuld occur in every 15 minutes.
Schedule here is 9:00 AM to 1 PM
Interval is every 15 minutes.