Namespace Planning in Exchange 2016

If you are like the vast majority of our customers, you already have some version(s) of Exchange deployed in your environment. Depending on the version, you may have different namespace requirements today.

Exchange 2010

Exchange 2010 leverages the Autodiscover service for enabling client profile changes, so that namespace exists.

Exchange 2010 introduced additional namespace requirements, which resulted in additional complexity around namespace planning, especially for site resilient solutions:

  1. Primary datacenter Internet protocol namespace (
  2. Secondary datacenter Internet protocol namespace (
  3. Primary datacenter Outlook Web App failback namespace (
  4. Secondary datacenter Outlook Web App failback namespace (
  5. Transport namespace (
  6. Primary datacenter RPC Client Access namespace (
  7. Secondary datacenter RPC Client Access namespace (

Out of these seven namespaces, five of them were required on certificates. The RPC Client Access namespaces were not required on the certificate because they were accessed via RPC connectivity and not via an Internet-based protocol, like HTTP.

Exchange 2016

One of the benefits of the Exchange 2016 architecture (first introduced in Exchange 2013) is that the namespace model can be simplified, when compared to Exchange 2010.

An example of how it can be simplified can be seen when thinking about a site resilience scenario. If you have two datacenters participating in a site resilient architecture, by replacing the Exchange 2010 infrastructure with Exchange 2016, five namespaces could potentially be removed:

  1. Secondary datacenter Internet protocol namespace (
  2. Primary datacenter Outlook Web App failback namespace (
  3. Secondary datacenter Outlook Web App failback namespace (
  4. Primary datacenter RPC Client Access namespace (
  5. Secondary datacenter RPC Client Access namespace (

There’s two reasons for this.

First, Exchange 2016 no longer leverages an RPC Client Access namespace.This is due to the architectural changes within the product – for a given mailbox, the protocol that services the request is always going to be the protocol instance on the Mailbox server that hosts the active copy of the database for the user’s mailbox. In other words, the RPC Client Access service is no longer decoupled from the store, like it was in Exchange 2010.

Second, as mentioned, the Client Access services proxies requests to the Mailbox server hosting the active database copy.

Figure 1: Client Access services (on MBX Server 1) proxying traffic to the Mailbox server hosting the active database copy (on MBX Server 3)

This proxy logic is not limited to the Active Directory site boundary. Unlike Exchange 2010, Exchange 2016 does not require the client namespaces to move with the DAG during an activation event – a Mailbox server in one Active Directory site can proxy a session to a Mailbox server that is located in another Active Directory site. This means that unique namespaces are no longer required for each datacenter ( and; instead, only a single namespace is needed for the datacenter pair – This also means failback namespaces are also not required during DAG activation scenarios, so and are removed.

Namespace Models

Depending on your architecture and infrastructure you have two choices:

  1. Deploy a unified namespace for the site resilient datacenter pair (unbound model).
  2. Deploy a dedicated namespace for each datacenter in the site resilient pair (bound model).

It’s also worth mentioning that these choices are also tied to the DAG architecture.

Unbound Model

In an unbound model, you have a single DAG deployed across the datacenter pair. This DAG has Mailbox servers in each datacenter – typically all Mailbox servers are active and host active database copies, however you could deploy all active copies in a single datacenter. Mailboxes for both datacenters are dispersed across the mailbox databases within this DAG. In this model, clients can connect to both datacenters in the event there is a WAN failure – neither datacenter’s connectivity is a boundary, hence the term unbound. It does not guarantee, however, the connectivity provides users an equal experience; meaning one connection may provide a better user experience because it has lower latency or more bandwidth.

In an unbound model, a single namespace is preferred because either datacenter can service the user request. This means that from a load balancing perspective, the Exchange 2016 Mailbox servers in both datacenters participate in handling traffic, as seen in the following diagram, where VIP (virtual IP address) is the load balanced IP address associated with the namespace:

Figure 2: Single Namespace used across Site Resilient Datacenter Pair (Unbound Model)

As a result, for a given datacenter, the expectation is that 50% of the traffic will be proxied from the other datacenter.

Bound Model

As its name implies, in a bound model, users are associated (or bound) to a specific datacenter. In other words, there is preference to have the users operate out of one datacenter during normal operations and only have the users operate out of the second datacenter during failure events. There is also a possibility that users do not have equal connectivity to both datacenters. Typically, in a bound model, there are two DAGs deployed in the datacenter pair. Each DAG contains a set of mailbox databases for a particular datacenter; by controlling where the databases are mounted, you control connectivity.

In a bound model, multiple namespaces are preferred, two per datacenter (primary and failback namespaces), to prevent clients trying to connect to the datacenter where they may have no connectivity. Switchover to the other datacenter is a controlled event.

Figure 3: Multiple Namespaces used across Site Resilient Datacenter Pair (Bound Model)

Autodiscover Namespace

Exchange 2016 takes advantage of the Autodiscover service for client profile configuration; so the namespace remains in place.

Office Online Server Namespaces

The document collaboration features included in Outlook on the web require Office Online Server. In site resilient deployments, you want to deploy an Office Online Server farm in each datacenter that participates in the site resilient datacenter pair.This ensures that there is a local instance that can service the document collaboration requests for the local mailboxes and avoids cross-site proxy scenarios.

From a namespace perspective, this means that each datacenter in the site resilient datacenter pair requires a unique namespace for Office Online Server; in other words, the namespace model for Office Online Server is a bound model.The namespace model that is used by Office Online Server is mutually exclusive from the model used by Exchange, meaning that you can deploy Exchange using an unbound model, while utilizing a bound model for Office Online Server as seen in the following figure:

Figure 4: Office Online Server Namespaces (Bound Model) with an Exchange Unbound Model Namespace

As all the data serviced by Office Online Server is either stored in Exchange or SharePoint, during a datacenter outage, namespace manipulation steps are not required. For example, if we refer to the previous diagram – if the West datacenter fails, you don’t need to change the DNS record for the Office Online Server namespace in West and point it to the load balancer in East. This is due to the architecture of Exchange and Office Online Server. Any Exchange 2016 Mailbox server will always proxy the client’s request to the Mailbox server that hosts the user’s mailbox database. The Mailbox server hosting the user’s mailbox is responsible for generating the Office Online Server URL that is used by OWA. This URL is defined per-Mailbox server, thereby ensuring that any Office Online Server interactions are always local to the Mailbox server.

Internal vs. External Namespaces

Since the release of Exchange 2007, the recommendation is to deploy a split-brain DNS infrastructure for the Internet-based client namespaces.A split-brain DNS infrastructure enables different IP addresses to be returned for a given namespace based on where the client resides – if the client is within the internal network, the IP address of the internal load balancer is returned; if the client is external, the IP address of the external gateway/firewall is returned.

This approach simplifies the end-user experience – users only have to know a single namespace (e.g., to access their data, regardless of where they are connecting. A split-brain DNS infrastructure, also simplifies the configuration of the Exchange virtual directories, as the InternalURL and ExternalURL values within the environment can be the same value.

In the event that you do not deploy a split-brain DNS (also known as split-DNS) infrastructure, Exchange 2016 does allow you to specify different namespaces for internal clients vs. external clients for all clients.

Important: In the event that you are utilizing a split-brain DNS infrastructure, then you must utilize the same authentication value for both your internal and external Outlook Anywhere settings, or switch to use different names for Outlook Anywhere inside and out. Outlook gives priority to the internal settings over the external settings and since the same namespace is used for both, regardless of whether the client is internal or external, it will utilize only the internal authentication settings.

Regional Namespace

The concept of regional namespaces has existed since OWA debuted in 1997. A regional namespace is a way for clients to connect to the client access endpoint that is closest to the Mailbox servers hosting the data.

Use of a regional namespace does not necessarily mean you are restricted to a bound model, either. This is because depending on your infrastructure and network capabilities, you may choose to have a dedicated namespace for each datacenter pair. For example, your company may have a set of datacenters in North America and in Europe, and due to a desire to reduce cross-region network traffic, you deploy a dedicated namespace for each region (notice that within a region, the unbound model is used):

Figure 5: : Regional Namespaces coupled with Geo-DNS to Round-Robin between Datacenters within a Region

Namespaces and Active Directory Site Topologies

When planning your namespace architecture, it is important to understand that namespaces and authentication settings must be identical and/or consistent within an Active Directory site. For example, when Autodiscover generates a response to send to the client, it generates a list of internal URLs based on the virtual directory settings of the Mailbox servers located in the Active Directory where the mailbox is located. If you attempt to have multiple namespaces within a single Active Directory site, clients will be randomly directed to different namespaces. Likewise, setting different authentication settings within an Active Directory site will lead to different behaviors for the clients. In other words, you can only define different namespace and authentication settings between Active Directory sites, not within Active Directory sites.


Exchange 2016 introduces significant flexibility in your namespace architecture, enabling deployment of a single unified namespace for a site resilient datacenter pair (or worldwide), or deployment of multiple namespaces.  As we delve into the intricacies surrounding load balancing principles and client connectivity, you will understand (hopefully) how to choose the best namespace model.

Ross Smith IV
Principal Program Manager
Office 365 Customer Experience

Comments (5)
  1. StephenKCEE says:

    Thanks Ross for keeping us up to date with Namespace planning, always fun!

  2. Great article! So basically nothing changes from Exchange 2013 to 2016, except for the introduction of Office Online Server namespaces. Thanks for the reminder of the authentication settings for Outlook Anywhere because Outlook will try the Internal settings

    "The concept of regional namespaces has existed since OWA debuted in 1997." Shouldn’t that read EWA? :-)

  3. Ravikumar Sathyamurthy (ShakthiRavi) says:

    Nice post !!!

  4. Thanks for clarification on how to plan Exchange 2016 namespaces

  5. Satyajit321 says:

    Nice article.
    I have a question though. "Any Exchange 2016 Mailbox server will always proxy the client’s request to the Mailbox server that hosts the user’s mailbox database", does it proxy to the CAS component or the Mailbox component of the destination server incase of
    Unbound model.

    In 2013 "6.CAS proxies the request to the Mailbox server hosting the active copy. ", it doesn’t matter though, still wanted to know.

    In similar grounds Ex2013 CAS to Ex2016 proxy will be thru what and vice versa. As I have read its more compatible in doing that.

    In Ex2013 ->Ex2010 it will be proxy but through CAS2013->CAS2010->MBX2010

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