2. How Site-Based Routing Works
Mailbox and Unified Messaging servers use site membership information to determine which Hub
Transport servers are located in the same site. This allows the Mailbox
or Unified Messaging server to submit messages for routing and
transport to a Hub Transport server that has the same site membership.
When a 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 site membership of
that Mailbox server. If the Client Access server is not 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.
Normally, Hub Transport servers retrieve information from Active
Directory to determine how they should transport mail inside the
organization. When a user sends a message, the Categorizer
running on the Hub Transport server uses the header information about
the message to query Active Directory for information about where the
server must deliver the message. If the recipient's mailbox is located
on a Mailbox server in the same site as the Hub Transport server, the
server delivers the message directly to that mailbox. If the recipient's
mailbox is located on a Mailbox server in a different site, the message
is transferred to a Hub Transport server in that site and then that
server delivers the message to the mailbox.
Exchange servers determine site membership by matching their assigned
IP address to a subnet that is defined in Active Directory Sites And
Services and associated with an Active Directory site. The Exchange
server then uses this information to determine which domain controllers,
Global Catalog servers, and other Exchange servers exist in that site,
and it communicates with those directory servers for authentication,
authorization, and messaging purposes. Exchange 2010 always tries to
retrieve information about recipients from directory servers that are in
the same site as the Exchange 2010 server.
Tip
In Active Directory, you can associate a site with one or more IP
subnets. Each subnet that is part of a site should be connected over
reliable, high-speed links. You should configure any business locations
connected over slow or unreliable links as part of separate sites.
Because of this, individual sites typically represent well-connected
local area networks (LANs) within an organization, and wide area network
(WAN) links between business locations typically mark the boundaries of
these sites. Sites cannot have overlapping subnet configurations. If subnets overlap, replication and message routing will not work correctly.
How IP Site Links Are Used
As Figure 1 shows, Active Directory sites are connected through IP site
links. An IP site link can connect two or more sites. Each site link
has a specific schedule, interval, and cost. The schedule and interval
determine the frequency of Active Directory replication.
The cost value determines the cost of using the link relative to other
links that might be available. Active Directory replication uses the
link with the lowest cost when multiple paths exist to a destination.
The cost of a route is determined by adding together the cost of all site
links in a transmission path. Administrators assign the cost value to a
link based on relative network speed, available bandwidth, and
reliability compared to other available connections. By default, IP site links always allow traffic to flow into or out of a site.
In large enterprises, message traffic might have to travel through
multiple sites to get from the source site to a destination site. When
transferring messages from one site to another site through other sites,
a Hub Transport server always tries to connect directly to a Hub
Transport server in the destination site. Because of this, messages are
not relayed through each Hub Transport server in each site in the link
path. Instead, they go directly from the Hub Transport server in the
originating site across the link to the Hub Transport server in the
destination site. If the originating server cannot connect directly to a
Hub Transport server in the destination site,
the originating Hub Transport server uses the link cost to determine
the closest site at which to queue the message. This feature is called queue at point of failure.
The Hub Transport server can also use the site link information to optimize the routing of messages that users send to multiple
recipients. Here, the Hub Transport server expands a distribution list
and creates multiple copies of a message only when there are multiple
paths in the routing topology. This feature is called delayed fan-out.
Understanding On-Premises, Online, and Cross-Premises Routing
Microsoft introduced Exchange
Online with Exchange Server 2007. Exchange Online is what's referred to
as a cloud service, meaning the service is provided via the Internet.
Exchange Online allows you to outsource all or part of your Exchange
services. Exchange Online differs from Exchange
on-premises (the standard implementation) in several fundamental ways.
With Exchange Online, the Exchange hardware resides elsewhere and users
access their mailboxes over the Internet. However, administrators still
retain control and management over the outsourced mailboxes.
In Exchange Server 2007, the on-premises and online Exchange
configurations weren't tightly integrated. Starting with Exchange Server
2010, Microsoft corrects this deficiency by making it possible to
manage both online and on-premises Exchange configurations using the
same set of management tools. You can simultaneously connect to and
manage both online and on-premises configurations in the Exchange
Management Console.
Although Exchange Online has some advantages over an Exchange
on-premises implementation, it has disadvantages as well. For users,
Exchange Online provides:
For administrators, Exchange Online provides:
-
Service Level Agreements
-
Storage quotas
-
Automatic backups
-
Automatic archiving
What Exchange Online doesn't provide is immediacy of access. Users
must always be connected to the Internet to get their mail. Messages
typically are routed and transferred across the Internet, which can
cause delays. Exchange Online also does not have Exchange voice mail,
custom transport rules, and some other features.
When you configure your Exchange organization, it's important to keep
in mind that Exchange Online is not an all-or-nothing implementation.
You can host some mailboxes online and others on premises—and Exchange Server 2010 makes it easy to manage mailboxes regardless of
where they are located. Before you transition mailboxes off-site,
however, you'll probably want to perform a trial with a limited subset
of users while keeping mailboxes for executives and most managers in
house. In fact, you might want to plan to always keep mailboxes for
executives and other high-level managers in house.
Exchange Server 2010 uses cross-premises
routing to transfer messages between on-premises and hosted mailboxes.
If you send a message to a user with a hosted mailbox, your
organization's transport servers will route the message across the
Internet to the hosted Exchange server. If you send a message to a user
with an on-premises mailbox, your organization's transport servers will
route the message across your organization to the appropriate Exchange
server.
Exchange provides features for migrating
mailboxes from online to on-premises environments and vice versa.
During the migration, a mailbox might exist in both locations
temporarily. When Exchange completes the migration, the mailbox exists
only in the destination environment. Outlook 2007 and later include an
Autodiscover feature that automatically connects messaging clients to
the correct Exchange server. This feature uses the user's SMTP e-mail
address during automatic discovery to determine where the mailbox is
currently located.
Normally, Autodiscover works very well. However, a conflict could occur if a user has a mailbox in Exchange Online and a mailbox in Exchange
on-premises or a user has the same primary SMTP e-mail address in
Exchange Online and Exchange on-premises. In these scenarios, the
Autodiscover feature normally does not configure Outlook for the
Exchange Online environment and instead uses Exchange on-premises. This
occurs because Exchange on-premises has priority over Exchange Online
when there is a conflict and the user's computer is connected to the
Active Directory domain. To resolve the problem, delete the original
mailbox from its original location as soon as possible after a mailbox
migration. If a user needs both an on-line and on-premises mailbox, do
not use the same primary SMTP e-mail address for both Exchange Online
and Exchange on-premises.
