An Active Directory site is a physical subnet that is connected using a high-speed connection. Active directory sites are connected using site links, which are low-bandwidth, unreliable connections.
Windows 2003 creates one default site link “DEFAULTIPSITELINK”, , which can be used for a site-to-site connection between two sites. “DEFAULTIPSITELINK” can be renamed in the Active Directory Sites and Services snap-in . Administrator can create additional site links using the Active Directory Sites and Services snap-in.
Since site links are used over low-bandwidth WAN links, the primary consideration when configuring site links should be is bandwidth usage. By default, replication is scheduled to occur over the site link 24 hours a day, 7 days a week, at an interval of 180 minutes. If you have limited bandwidth, you should consider altering this.
When multiple links are configured between sites, priority of each link should be considered. You should assign priority of link based on availability and reliability of the connection. The default link cost is 100, and if many links to a site, the link with the lowest cost is used first.
You can use any of two transport protocols with site links.
Directory Service Remote Procedure Call (DS-RPC)
DS-RPC can be used when there is a live, reliable connection between two or more domain controllers in different sites. IP site links communicate synchronously, meaning each replication transaction must complete before another can start. By default, intersite IP replication adheres to replication schedules and does not require a certificate authority (CA).
Inter-Site Messaging Simple Mail Transport Protocol (ISM-SMTP)
SMTP replication can be used when the network connections are unreliable. SMTP site links communicate asynchronously, which means each replication transaction does not need to complete before another can start. Schedules are not available for SMTP replication and requires CA to sign SMTP messages for the authenticity of directory updates.
Important Notes to remember
• Intrasite replication always uses RPC over IP.
• Intersite replication can use either RPC over IP or SMTP.
• Intersite replication using SMTP is supported only for domain controllers in different domains. Domain controllers in the same domain must replicate using RPC over IP