Topologies for Azure AD Connect

The objective of this topic is to describe different on-premises and Azure AD topologies with Azure AD Connect sync as the key integration solution. It describes both supported and unsupported configurations.

Legend for pictures in the document:

Description Icon
On-premises Active Directory forest AD
Active Directory with filtered import AD
Azure AD Connect sync server Sync
Azure AD Connect sync server “Staging mode” Sync
GALSync with FIM2010 or MIM2016 Sync
Azure AD Connect sync server, detailed Sync
Azure AD directory AAD
Unsupported scenario Unsupported

Single forest, single Azure AD tenant

Single Forest Single Tenant

The most common topology is a single forest on-premises, with one or multiple domains, and a single Azure AD tenant. For Azure AD authentication, password synchronization is used. The express installation of Azure AD Connect supports only this topology.

Single forest, multiple sync servers to one Azure AD tenant

Single Forest Filtered Unsupported

It is not supported to have multiple Azure AD Connect sync servers connected to the same Azure AD tenant, except for a staging server. It is unsupported even if these are configured to synchronize mutually exclusive set of objects. You might have considered this if you cannot reach all domains in the forest from a single server or to distribute load across several server.

Multiple forests, single Azure AD tenant

Multi Forest Single Tenant

Many organizations have environments with multiple on-premises Active Directory forests. There are various reasons for having more than one on-premises Active Directory forest. Typical examples are designs with account-resource forests and as a result after a merger or acquisition.

When you have multiple forests, all forests must be reachable by single Azure AD Connect sync server. You do not have to join the server to a domain. If necessary to reach all forest, the server can be placed in a network DMZ.

The Azure AD Connect installation wizard offers several options to consolidate users represented in multiple forests. The goal is that a user is only represented once in Azure AD. There are some common topologies that you can configure in the custom installation path in the installation wizard. Select the corresponding option representing your topology on the page Uniquely identifying your users. The consolidation is only configured for users. Duplicated groups are not consolidated with the default configuration.

Common topologies are discussed in the next section: Separate topologies, Full mesh, and Account-Resource.

The default configuration in Azure AD Connect sync assumes:

  1. Users have only one enabled account and the forest where this account is located is used to authenticate the user. This assumption is for both password sync and for federation. UserPrincipalName and sourceAnchor/immutableID come from this forest.
  2. Users have only one mailbox.
  3. The forest that hosts the mailbox for a user has the best data quality for attributes visible in the Exchange Global Address List (GAL). If there is no mailbox on the user, then any forest can be used to contribute these attribute values.
  4. If you have a linked mailbox, then there is also another account in a different forest used for sign in.

If your environment does not match these assumptions, the following happens:

  • If you have more than one active account or more than one mailbox, the sync engine picks one and ignore the other.
  • A linked mailbox with no other active account is not exported to Azure AD. The user account is not represented as a member in any group. A linked mailbox in DirSync would always be represented as a normal mailbox. This change is intentionally a different behavior to better support multi-forest scenarios.

More details can be found in understanding the default configuation.

Multiple forests, multiple sync servers to one Azure AD tenant

Multi Forest Multi Sync Unsupported

It is not supported to have more than one Azure AD Connect Sync server connected to a single Azure AD tenant. The exception is the use of a staging server.

Multiple forests – separate topologies

Users are represented only once across all directories

Multi Forest Users Once

Multi Forest Seperate Topologies

In this environment, all forests on-premises are treated as separate entities and no user would be present in any other forest. Each forest has its own Exchange organization and there is no GALSync between the forests. This topology could be the situation after a merger/acquisition or in an organization where each business unit is operating isolated from each other. These forests are in the same organization in Azure AD and appear with a unified GAL. In this picture, each object in every forest is represented once in the metaverse and aggregated in the target Azure AD tenant.

Multiple forests – match users

User identities exist across multiple directories

Common for all these scenarios is that distribution and security groups can contain a mix of users, contacts, and FSPs (Foreign Security Principals)

FSPs are used in ADDS to represent members from other forests in a security group. All FSPs are resolved to the real object in Azure AD.

Multiple forests – full mesh with optional GALSync

User identities exist across multiple directories. Match using: Mail attribute

Multi Forest Users Mail

Multi Forest Full Mesh

A full mesh topology allows users and resources to be located in any forest and commonly there would be two-way trusts between the forests.

If Exchange is present in more than one forest, there could optionally be an on-premises GALSync solution. Every user would be represented as a contact in all other forests. GALSync is commonly implemented using Forefront Identity Manager 2010 or Microsoft Identity Manager 2016. Azure AD Connect cannot be used for on-premises GALSync.

In this scenario, identity objects are joined using the mail attribute. A user with a mailbox in one forest is joined with the contacts in the other forests.

Multiple Forests – Account-Resource Forest

User identities exist across multiple directories. Match using: ObjectSID and msExchMasterAccountSID attributes

Multi Forest Users ObjectSID

Multi Forest AccountResource

In an account-resource forest topology, you have one or more account forests with active user accounts. You also have one or more resource forests with disabled accounts.

In this scenario one (or more) resource forest trusts all account forests. The resource forest has typically an extended AD schema with Exchange and Lync. All Exchange and Lync services as well as other shared services are located in this forest. Users have a disabled user account in this forest and the mailbox is linked to the account forest.

Office 365 and topology considerations

Some Office 365 workloads have certain restrictions to supported topologies. If you plan to use any of these, then read the supported topologies topic for the workload.

Exchange Online If there is more than one Exchange organization on-premises (i.e. Exchange has been deployed to more than one forest) then you must use Exchange 2013 SP1 or later. Details can be found here: Hybrid deployments with multiple Active Directory forests
Skype for Business When using multiple forests on-premises then only the account-resource forest topology is supported. Details for supported topologies can be found here: Environmental requirements for Skype for Business Server 2015

Staging server

Staging Server

Azure AD Connect supports installing a second server in Staging mode. A server in this mode reads data from all connected directories but does not write anything to connected directories. It is using the normal synchronization cycle and therefore has an updated copy of the identity data. In a disaster where the primary server fails you can fail over to the staging server. You do this in the Azure AD Connect wizard. This second server can preferably be located in a different datacenter since no infrastructure is shared with the primary server. You must manually copy any configuration change made on the primary server to the second server.

A staging server can also be used to test a new custom configuration and the effect it has on your data. You can preview the changes and adjust the configuration. When you are happy with the new configuration, you can make the staging server the active server and set the old active server in staging mode.

This method can also be used to replace the active sync server. Prepare the new server and set it in staging mode. Make sure it is in good state, disable staging mode (making it active), and shut down the currently active server.

It is possible to have more than one staging server when you want to have multiple backups in different data centers.

Multiple Azure AD tenants

Microsoft recommends having a single tenant in Azure AD for an organization. Before you plan to use multiple Azure AD tenants, these topics cover common scenarios allowing you to use a single tenant.

Delegation using administrative units Administrative units management in Azure AD

Multi Forest Multi tenant

There is a 1:1 relationship between an Azure AD Connect sync server and an Azure AD tenant. For each Azure AD tenant, you need one Azure AD Connect sync server installation. The Azure AD tenant instances are by design isolated and users in one cannot see users in the other tenant. If this separation is intended, then this is a supported configuration, but otherwise you should use the single Azure AD tenant model.

Each object only once in an Azure AD tenant

Single Forest Filtered

In this topology, one Azure AD Connect sync server is connected to each Azure AD tenant. The Azure AD Connect sync servers must be configured for filtering so each have a mutually exclusive set of objects to operate on. You can for example scope each server to a particular domain or OU. A DNS domain can only be registered in a single Azure AD tenant. The UPNs of the users in the on-premises AD must use separate namespaces as well. For example, in the picture above three separate UPN suffixes are registered in the on-premises AD:,, and The users in each on-premises AD domain use a different namespace.

There is no GALsync between the Azure AD tenant instances. The address book in Exchange Online and Skype for Business only shows users in the same tenant.

This topology has the following restrictions to otherwise supported scenarios:

  • Only one of the Azure AD tenants can enable Exchange hybrid with the on-premises Active Directory.
  • Windows 10 devices can only be associated with one Azure AD tenant.
  • The SSO option for password synchronization and pass-through authentication can only be used with one Azure AD tenant.

The requirement for mutually exclusive set of objects also applies to writeback. Some writeback features are not supported with this topology since these features assume a single configuration on-premises:

  • Group writeback with default configuration
  • Device writeback

Each object multiple times in an Azure AD tenant

Single Forest Multi Tenant Unsupported Single Forest Multi Connectors Unsupported

  • It is unsupported to sync the same user to multiple Azure AD tenants.
  • It is unsupported to make a configuration change to make users in one Azure AD appear as contacts in another Azure AD tenant.
  • It is unsupported to modify Azure AD Connect sync to connect to multiple Azure AD tenants.

GALsync by using writeback

MultiForestMultiDirectoryGALSync1Unsupported MultiForestMultiDirectoryGALSync2Unsupported

Azure AD tenants are by design isolated.

  • It is unsupported to change the configuration of Azure AD Connect sync to read data from another Azure AD tenant.
  • It is unsupported to export users as contacts to another on-premises AD using Azure AD Connect sync.

GALsync with on-premises sync server


It is supported to use FIM2010/MIM2016 on-premises to GALsync users between two Exchange organizations. The users in one organization shows up as foreign users/contacts in the other organization. These different on-premises ADs can then be synchronized to their own Azure AD tenants.

Next steps

To learn how to install Azure AD Connect for these scenarios, see Custom installation of Azure AD Connect.

Learn more about the Azure AD Connect sync configuration.

Learn more about Integrating your on-premises identities with Azure Active Directory.