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Planning to Use Active Directory Sites for Routing Mail

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Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2

Microsoft Exchange Server 2010 uses Active Directory site topology to determine how messages are transported in the organization. Exchange 2010 uses the existing Active Directory site topology to transport messages between server roles.

The Hub Transport server role provides message transport inside the Exchange organization. When you're deploying a pure Exchange 2010 organization, or introducing Exchange 2010 into a pure Exchange Server 2007 organization, no additional configuration is required to establish routing in the forest. If you're deploying Exchange 2010 in an existing Exchange Server 2003 organization, you must follow specific configuration steps to enable routing between Exchange 2010 and Exchange 2003. For more information about how to configure the Hub Transport server role for coexistence with Exchange 2003, see Upgrade from Exchange 2003 Transport.

Contents

How Exchange 2010 Uses Site Membership

Determining Site Membership

Overview of IP Site Links

Exchange 2010 Placement in Active Directory Sites

How Exchange 2010 Uses Site Membership

Exchange 2010 is a site-aware application. Site-aware applications can determine their own Active Directory site membership and the Active Directory site membership of other servers by querying Active Directory. Exchange 2010 uses site membership to determine which domain controllers and global catalog servers to use for processing Active Directory queries. Additionally, when a server running Exchange has to determine the Active Directory site membership of another Exchange server, it can query Active Directory to retrieve the site name.

In Exchange 2010, the Microsoft Exchange Active Directory Topology service is responsible for updating the site attribute of the Exchange server object. Because the Active Directory site membership is a server object attribute, Exchange doesn't have to query the Domain Name System (DNS) to resolve a server address to a subnet associated with an Active Directory site. Stamping the Active Directory site attribute on an Exchange server object also enables Active Directory site membership to be assigned to a server that isn't a domain member, such as a subscribed Edge Transport server.

The Exchange 2010 server roles use Active Directory site membership information as follows:

  • Mail submission   The Mailbox server role uses Active Directory site membership information to determine which Hub Transport servers are located in the same Active Directory site as the Mailbox servers with the same server version. The Mailbox server submits messages for routing and transport to a Hub Transport server that has the same Active Directory site membership and the same server version as the Mailbox server.

  • Mail delivery   The Hub Transport server performs recipient resolution and queries Active Directory to match an e-mail address to a recipient account. The recipient account information includes the fully qualified domain name (FQDN) of the user's Mailbox server. The Hub Transport server queries Active Directory to determine the Active Directory site of the user's Mailbox server. If the Mailbox server is in the same site as the Hub Transport server, it will deliver the message to that Mailbox server. Otherwise, it will relay the message to another Hub Transport server in the same site as the target Mailbox server for delivery.

  • Message routing   Exchange 2010 Hub Transport servers retrieve information from Active Directory to determine how mail should be routed inside the organization. When a message is submitted to the Microsoft Exchange Transport service, the categorizer uses the header information in the message to query Active Directory for information about where the message must be delivered. If the recipient's mailbox is located on a Mailbox server in the same Active Directory site as the Hub Transport server and the version of the Mailbox server matches the Hub Transport server, the message is delivered directly to that mailbox. If the recipient's mailbox is located on a Mailbox server that has a different server version than the Hub Transport server, the message is relayed to a Hub Transport server in the site that matches the version of the Mailbox server. If the recipient's mailbox is located on a Mailbox server in a different Active Directory site, the message is relayed to a Hub Transport server in that site and then delivered to the Mailbox server.

  • Unified Messaging message submission   The Unified Messaging server role uses Active Directory site membership information to determine which Hub Transport servers are located in the same Active Directory site as the Unified Messaging server. The Unified Messaging server submits messages for routing to a Hub Transport server within the same Active Directory site. The Hub Transport server performs recipient resolution and queries Active Directory to match a telephone number, or another Unified Messaging property, to a recipient account. After the recipient resolution completes, the Hub transport server will deliver the message to the target mailbox in the same way as a regular e-mail message.

  • Client connections to Client Access server   When the Client Access server receives a user connection request, it queries Active Directory to determine which Mailbox server is hosting the user's mailbox. The Client Access server then retrieves the Active Directory site membership of that Mailbox server. If the Client Access server that received the initial user connection isn't located in the same site as the user's Mailbox server, the connection is redirected to a Client Access server in the same site as the Mailbox server.

  • Public folder referrals   Active Directory site membership and IP site link information is used to prioritize the list of servers used for public folder referrals. Users are directed first to the default public folder database for their mailbox database. If a replica of the public folder being accessed doesn't exist in the default public folder database, the Mailbox store where the default public folder database resides will provide a prioritized referral list of Mailbox servers that hold a replica to the client. Public folder databases in the same Active Directory site as the default public folder database are listed first, and additional referral locations are prioritized based on Active Directory site proximity. Active Directory site proximity is determined by aggregating the costs of the IP site links between the Active Directory site where the default public folder database resides and the Active Directory sites where public folder replicas exist. The list of referrals is prioritized from lowest cost to highest cost. The connecting client will try each referral in the list until a connection is made or all attempts fail.

Determining Site Membership

Active Directory clients assume site membership by matching their assigned IP address to a subnet defined in Active Directory Sites and Services and associated with an Active Directory site. The client then uses this information to determine which domain controllers and global catalog servers exist in that site and communicates with those directory servers for authentication and authorization purposes. Exchange 2010 takes advantage of this relationship by also preferring to retrieve information about recipients from directory servers in the same site as the Exchange 2010 server.

All computers that are part of the same Active Directory site are considered well connected, with a high-speed, reliable network connection. By default, when an Active Directory forest is first deployed, there's a single site named Default-First-Site-Name. If no other sites are manually configured by the administrator, all server and client computers in the forest are considered members of Default-First-Site-Name.

When more than one site is defined, the Active Directory administrator must define the subnets present in the organization and associate those subnets with Active Directory sites.

The Microsoft Exchange Active Directory Topology service checks the site membership attribute on the Exchange server object when the server starts. If the site attribute has to be updated, the Microsoft Exchange Active Directory Topology service stamps the attribute with the new value. The Microsoft Exchange Active Directory Topology service verifies the site attribute value every 15 minutes and updates the value if site membership has changed. The Microsoft Exchange Active Directory Topology service uses the Net Logon service to obtain current site membership. The Net Logon service updates site membership every five minutes. This means that up to a 20 minute latency period may pass between the time that site membership changes and the new value is stamped on the site attribute.

Relationships between Active Directory sites are defined by IP site links. The IP site link consists of two or more Active Directory sites. All Active Directory sites that are part of the link communicate at the same cost. The IP site link properties include a cost assignment, a schedule, and an interval. The schedule and interval properties are only used for determining Active Directory replication frequency. Exchange 2010 uses the cost assignment to determine the lowest cost route for traffic to follow when multiple paths exist to the destination. The cost of the route is determined by aggregating the cost of all site links in a transmission path. The Active Directory administrator assigns the cost to a link based on relative network speed and available bandwidth compared to other available connections.

By default, the Hub Transport server always tries a direct connection to a Hub Transport server in another Active Directory site. Messages in transport don't relay through each Hub Transport server in a site link path. However, Hub Transport servers in intermediate Active Directory sites along the routing path may perform message relay in the following scenarios:

  • Direct relay between Hub Transport servers won't occur when a hub site exists along the least cost routing path. You can configure an Active Directory site as a hub site so that messages are routed to the hub site to be processed before the messages are relayed to the target server. Hub sites are discussed later in this topic.

  • Exchange 2010 uses the routing path derived from IP site link information when communication to the destination Active Directory site fails. If no Hub Transport server in the destination Active Directory site responds, message delivery backs off along the least cost routing path until a connection is made to a Hub Transport server in an Active Directory site along the routing path. The messages are queued in that Active Directory site and the queue will be in a retry state. This behavior is called queue at point of failure.

  • The Hub Transport server can also use the IP site link information to optimize routing of messages sent to multiple recipients. The Hub Transport server delays bifurcation of messages until it reaches a fork in the routing paths to the recipients. The bifurcated message is relayed to each recipient destination by a Hub Transport server in the Active Directory site that represents the fork in the individual routing paths. This functionality is called delayed fan-out.

Designating Hub Sites

By default, the Hub Transport servers located in Active Directory sites along the path between the source server and the destination server don't process or relay the messages. You can use the Set-AdSite cmdlet to override this behavior by configuring an Active Directory site as a hub site. When a hub site exists along the least cost routing path between two Hub Transport servers, the messages are routed to the hub site for processing before they are relayed to the destination server. For this routing behavior to occur, the hub site must exist along the least cost routing path between two Hub Transport servers. This configuration should only be used when it's required by the network topology, such as when firewalls exist between Active Directory sites and prevent direct relay of SMTP communications.

You can use the Set-AdSiteLink cmdlet in the Exchange Management Shell to configure an Exchange-specific cost to an Active Directory IP site link. The Exchange-specific cost is a separate attribute used instead of the Active Directory-assigned cost to determine the Exchange routing path. This configuration is useful when the Active Directory IP site link costs don't result in an optimal Exchange message routing topology.

By default, Exchange 2010 doesn't impose a maximum message size limit on messages relayed between Hub Transport servers in different Active Directory sites. If you use the Set-AdSiteLink cmdlet to configure a maximum message size on an Active Directory IP site link, routing generates a non-delivery report (NDR) for any message that has a size larger than the maximum message size limit configured on any Active Directory site link in the least cost routing path. This configuration is useful for restricting the size of messages sent to remote Active Directory sites that must communicate over low-bandwidth connections.

Exchange 2010 Placement in Active Directory Sites

For message routing between Exchange 2010 roles to occur correctly, all roles deployed in the forest must belong to an Active Directory site. Make sure that the IP addresses that you have assigned are in subnets that are correctly associated with Active Directory sites.

The first step in planning the placement of Exchange 2010 servers in the Active Directory site topology is to document the current topology. Your documentation should include the following:

  • Sites

  • Subnets and their site association

  • IP site links and their member sites

  • IP site link costs

  • Directory servers in each site

  • Physical network connections

  • Firewall locations

After you have diagrammed these objects, plan the placement of Exchange servers. Consider the following information when deciding where to put servers:

  • A Hub Transport server must be able to communicate directly with a global catalog server to perform Active Directory lookups.

  • Mailbox servers should be located in the same site as a Hub Transport server. We recommend that you deploy more than one Hub Transport server in each Active Directory site to provide load balancing and fault tolerance.

  • Unified Messaging servers submit messages to a Hub Transport server for transport to a Mailbox server. A Unified Messaging server may be located in a hub site or near the IP or Voice over IP (VoIP) gateway or IP Private Branch eXchange (IP PBX). The Hub Transport server that has the same site membership as the Unified Messaging server will receive messages for transport and route the messages to other Hub Transport servers and Mailbox servers in the organization.

  • Client Access servers provide a connectivity point to the Exchange organization for users who are accessing Exchange remotely. A Client Access server must be deployed in each site that contains Mailbox servers.

After you plan Exchange 2010 server placement, you may identify areas where you can modify the Active Directory site topology to improve communication flow. You may want to adjust IP site links and site link costs to optimize delayed fan-out and queue at point of failure. An efficient Active Directory topology doesn't require any changes to support Exchange 2010.

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