What is Active Directory replication?

Replication must often occur both (intrasite) within sites and (Intersite) between sites to keep domain and forest data consistent among domain controllers that store the same directory partitions.

Intrasite replication or Replication within site:
The KCC creates separate replication topologies to transfer Active Directory updates within a site and between all configured sites in the forest. The connections that are used for replication within sites are created automatically with no additional configuration. Intrasite replication takes advantage of LAN network speeds by providing replication as soon as changes occur, without the overhead of data compression, thus maximizing CPU efficiency. Intrasite replication connections form a ring topology with extra shortcut connections where needed to decrease latency. The fast replication of updates within sites facilitates timely updates of domain data. In deployments where large datacenters constitute hub sites for the centralization of mission-critical operations, directory consistency is critical.

Intersite Replication or Replication between sites:
Replication between sites is made possible by user-defined site and site link objects that are created in Active Directory to represent the physical LAN and WAN network infrastructure. When Active Directory sites and site links are configured, the KCC creates an intersite topology so that replication flows between domain controllers across WAN links. Intersite replication occurs according to a site link schedule so that WAN usage can be controlled, and is compressed to reduce network bandwidth requirements. Site link settings can be managed to optimize replication routing over WAN links. The connections that are created between sites form a spanning tree for each directory partition in the forest, merging where common directory partitions can be replicated over the same connection.


What is FRS?
File Replication service (FRS) is related to Active Directory replication because it requires the Active Directory replication topology. FRS is a multimaster replication service that is used to replicate files and folders in the system volume (SYSVOL) shared folder on domain controllers and in Distributed File System (DFS) shared folders. FRS works by detecting changes to files and folders and then replicating the updated files and folders to other replica members, which are connected in a replication topology.

FRS uses the replication topology that is generated by the KCC to replicate the SYSVOL files to all domain controllers in the domain. SYSVOL files are required by all domain controllers for Active Directory to function.

Introduction to and Administration of the File Replication Service

FRS WEBCAST, FRS VIDEO, FRS TUTORIAL, TROUBLESHOOTING FRS


What are the two protocols that are used in replication?
RPC over IP and SMTP over IP.

SMTP
Simple Mail Transfer Protocol (SMTP) is a packaging protocol that can be used as an alternative to the remote procedure call (RPC) replication transport. SMTP can be used to transport nondomain replication over IP networks in mail-message format. Where networks are not fully routed, e-mail is sometimes the only transport method available


Replication transports provide the wire protocols that are required for data transfer. There are three levels of connectivity for replication of Active Directory information:

• Uniform high-speed, synchronous RPC over IP within a site.

• Point-to-point, synchronous, low-speed RPC over IP between sites.

• Low-speed, asynchronous SMTP between sites.



The following rules apply to the replication transports:

• Replication within a site always uses RPC over IP.

• Replication between sites can use either RPC over IP or SMTP over IP.

• Replication between sites over SMTP is supported for only domain controllers of different domains. Domain controllers of the same domain must replicate by using the RPC over IP transport. Therefore, replication between sites over SMTP is supported for only schema, configuration, and global catalog replication, which means that domains can span sites only when point-to-point, synchronous RPC is available between sites.


Synchronous and Asynchronous Communication
The RPC intersite and intrasite transport (RCP over IP within sites and between sites) and the SMTP intersite transport (SMTP over IP between sites only) correspond to synchronous and asynchronous communication methods, respectively. Synchronous communication favors fast, available connections, while asynchronous communication is better suited for slow or intermittent connections.


KCC: It creates the replication topology within the site.
ISTG: It creates the topology for the replication between the sites of the same domain.
Bridgehead server: These servers are responsible to receive the receiving the replication data from another site and then replicate to the servers within the site. Any replication originating from its site will be sent to other sites by this server only.


What is FRS?
The File Replication service (FRS) is a multi-threaded, multi-master replication engine that replaces the LMREPL (LanMan Replication) service in the 3.x/4.0 versions of Microsoft Windows NT. Windows 2000 domain controllers and servers use FRS to replicate system policy and logon scripts for Windows 2000 and earlier clients that are located in the System Volume (Sysvol).
FRS can also replicate content between Windows 2000 servers hosting the same fault-tolerant Distributed File System (DFS) roots or child node replicas.


What is Journal Wrap?
Journal wrap errors occur if a sufficient number of changes take place while FRS is turned off such that the last USN change that FRS recorded during shutdown no longer exists in the USN journal during startup. The risk is that changes to files and folders for FRS replicated trees may have taken place while the service was turned off, and no record of the change exists in the USN journal. To guard against data inconsistency, FRS asserts into a journal wrap state.

Troubleshooting journal_wrap errors on Sysvol and DFS replica sets।

Tags : Active Directory FAQ, Active Directory, Active direcotry replication, intersite replication, intrasite replication, Journal Wrap, RPC over IP, SMTP over IP, FSMO roles