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Message Routing in a Coexistence Environment

 

Applies to: Exchange Server 2007 SP3, Exchange Server 2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007

Topic Last Modified: 2009-01-14

This topic describes how message routing occurs when Microsoft Exchange Server 2007 coexists in the same Exchange organization with Exchange Server 2003 or Exchange 2000 Server computers. Make sure that you understand the routing changes that are introduced by the addition of Exchange 2007 to an existing Exchange organization so that you can configure connectors and avoid routing loops. When a large organization is transitioning from Exchange 2003 to Exchange 2007, a period of coexistence between the versions is likely.

Exchange 2007 introduces routing changes that take advantage of the existing Active Directory directory service site topology and the underlying network to provide an efficient, deterministic routing topology. When Exchange 2007 coexists with Exchange 2003 or Exchange 2000, you must perform additional configuration tasks to support message routing between the server versions. Table 1 summarizes the changes in message routing between versions of Exchange Server.

Table 1   Routing differences between versions of Exchange Server

 

Exchange 2007 Exchange 2000 and Exchange 2003

Exchange uses Active Directory sites to determine an intra-organizational routing topology. All Exchange 2007 servers are associated with a single routing group for the purposes of routing to earlier versions of Exchange Server.

Exchange uses routing groups to determine an intra-organizational routing topology.

Exchange determines the least cost route between Hub Transport servers by using Active Directory directory service IP site link costs.

Exchange determines the least cost route between bridgehead servers by using routing group connector costs.

Exchange uses direct relay to deliver messages between Hub Transport servers.

Exchange relays through bridgehead servers in each routing group in the routing path.

When Exchange can't connect, it uses the least cost routing path information to back off from the destination until a connection can be made to a Hub Transport server. Messages queue at the reachable site that is closest to the destination. This behavior is known as queue at point of failure.

When Exchange can't connect to the next hop in a routing path, tries to reroute the message over an alternative path.

When a message is being sent to multiple recipients, Exchange delays message splitting until a fork in the routing path is reached. This behavior is known as delayed fan-out.

When a message is being sent to multiple recipients, message splitting occurs immediately after recipient resolution.

Each Hub Transport server queries Active Directory separately to retrieve the routing configuration used to calculate a routing table and to receive configuration updates.

Exchange uses a link state table to store a routing table and advertises configuration changes by using link state updates. The routing group master retrieves updates from Active Directory and coordinates the propagation of link state changes that are learned by servers in its routing group.

When the first Exchange 2007 server is installed in an existing Exchange organization, you are prompted to select a bridgehead server in the existing organization with which to establish the initial routing group connector. Exchange 2007 only uses routing group connectors when it communicates with Exchange 2003 or Exchange 2000 servers in the same Exchange organization. During Exchange 2007 setup, a routing group connector is created in both directions between the Exchange 2007 routing group and an Exchange 2003 or Exchange 2000 routing group. The Exchange 2003 or Exchange 2000 bridgehead server that you select during setup determines with which routing group the connection is made. After setup is complete, it is a best practice to add source and target servers to the routing group connectors for load balancing and redundancy.

All Exchange 2007 servers are automatically put in a single routing group that is called Exchange Routing Group (DWBGZMFD01QNBJR). Exchange 2007 servers and Exchange 2003 or Exchange 2000 servers cannot reside in the same routing group. You cannot create additional routing groups in which to put Exchange 2007 servers. The Exchange 2007 routing group is created strictly for coexisting with earlier versions of Exchange. The initial routing group connectors that are created during setup determine how messages flow between Exchange versions. The initial routing group connector is assigned a cost of 1. The Hub Transport server role that you installed and the Exchange 2003 or 2000 bridgehead server that you selected are set as the source and target servers. Permissions are granted to the bridgehead server to send e-mail to and receive e-mail from Exchange 2007 Hub Transport servers.

importantImportant:
Do not move Exchange 2007 servers out of Exchange Routing Group (DWBGZMFD01QNBJR) and do not rename Exchange Routing Group (DWBGZMFD01QNBJR) by using a low-level directory editor. Exchange 2007 must use this routing group to communicate with earlier versions of Exchange. We do not support moving Exchange 2007 servers out of Exchange Routing Group (DWBGZMFD01QNBJR) or renaming of Exchange Routing Group (DWBGZMFD01QNBJR).

The Exchange 2003 routing group to which you create your initial connection is important and depends on the structure of your current environment. Ideally, your routing groups mirror your Active Directory site structure, and your routing group connectors are in a hub-and-spoke format. In this scenario, your first Exchange 2007 deployment will be near the hub routing group. You should create your first connector to a bridgehead server in that routing group.

All messages that are relayed between Exchange 2007 and Exchange 2003 are routed through the initial routing group connector. This can create excessive routing hops as more Exchange 2007 servers are deployed in additional Active Directory sites. Exchange 2007 servers in all sites are considered members of the same routing group. For example, suppose you have routing groups in Hong Kong, London, and Chicago. If your first Exchange 2007 server is deployed in Chicago, it makes sense to establish the first routing group connector to a bridgehead server in Chicago. However, if you then deploy an Exchange 2007 server in Hong Kong, when messages are sent from users whose mailboxes are on Exchange 2003 servers in Hong Kong to users whose mailboxes are on Exchange 2007 servers in Hong Kong, the messages will be routed through Chicago.

To avoid such excessive routing hops, you can create another routing group connector that connects the single Exchange 2007 routing group to the Hong Kong routing group. In this scenario, you must make sure that you perform the configuration steps that avoid potential routing loops. We recommend that you transition all the Exchange 2003 servers in a routing group at the same time to avoid a routing topology that results in many hops.

To create a routing group connector that includes an Exchange 2007 Hub Transport server as either a source server or target server, you must use the New-RoutingGroupConnector cmdlet in the Exchange Management Shell. By default, a routing group connector that is created by using this cmdlet will have a default cost of 1 and will have public folder referrals enabled. To create the reciprocal routing group connector in a single operation, you must set the Bidirectional parameter is $True. For more information, see the following topics:

When only one routing group connector is established between Exchange 2003 and Exchange 2007, you do not have to make any changes to link state, and routing loops will not occur. However, if more than one routing group connector is configured between Exchange 2003 and Exchange 2007, the minor link state updates that are transmitted between Exchange 2003 servers can introduce problems. When Exchange 2003 detects that a connector is unavailable, link state updates are communicated throughout the Exchange organization to notify them of the connector down state. The Exchange 2003 bridgehead server also tries to determine an alternative route for message transfer to the destination server. However, Exchange 2007 does not use link state to determine a routing path. The Exchange 2007 Hub Transport server will be unaware of the down connector state and may decide to route a message back through a routing path that Exchange 2003 is trying to route around.

Exchange 2003 may try a routing path other than the least cost routing path when it detects that a connector is down. However, Exchange 2007 will always use the least cost route, introducing the potential for a routing loop to occur.

To avoid routing loops, you must suppress minor link state updates before introducing additional routing group connectors. Minor link state updates are sent between Exchange 2003 servers to update the link state routing table to indicate that a connector is down. When the SuppressStateChanges registry key is set, you are turning off the ability for a connector to be marked as down. Link state messages are also used to notify Exchange 2003 servers of configuration changes to the Exchange organization, such as the addition or removal of a connector or a server. When you suppress minor link state updates, it does not prevent these major link state update messages from being communicated.

When minor link state updates are suppressed, Exchange 2003 also only uses least cost routing. This eliminates the chance for routing loops to occur. We recommend that you suppress link state updates on every Exchange 2003 server in the organization to maintain a consistent configuration.

importantImportant:
If configuration changes are made in Exchange Routing Group (DWBGZMFD01QNBJR) some latency may occur before those changes are received by Exchange Server 2003 servers and propagated by the Exchange 2003 routing group masters. The delay will depend on how frequently the routing group masters poll for configuration changes in other routing groups. By default, the polling interval is set to one hour. To immediately register all changes in Exchange Routing Group (DWBGZMFD01QNBJR) on Exchange 2003 servers, you must restart the routing group masters.

Exchange 2003 and Exchange 2007 can both route to a connector that is hosted by either Exchange Server version. However, because of schema differences in connector configuration, some settings of the Send connector on an Exchange 2007 server will not be recognized by an Exchange 2003 server, and some settings on an SMTP connector on an Exchange 2003 server will not be recognized by an Exchange 2007 server. These differences can cause conflicts when the routing selection is made. Table 2 summarizes the differences in connector feature support between Exchange 2003 and Exchange 2007.

Table 2   Connector feature support

 

Connector feature Exchange Server version support Comment

Per user connector delivery restrictions

Exchange 2003

Exchange 2007 may route a message to an Exchange 2003 connector that does not allow connections from the sending user.

Message priorities

Exchange 2003

Exchange 2007 does not assign message priority and will bypass any priority restrictions set on an Exchange 2003 SMTP connector.

Message type (system and non-system designations)

Exchange 2003

Exchange 2007 does not assign message type and will bypass any message type restrictions set on an Exchange 2003 SMTP connector.

Connector scope

Exchange 2003 and Exchange 2007

Exchange 2003 and Exchange 2007 define connector scope differently. An Exchange 2003 connector can be scoped to only allow servers within the same routing group to use the connector. An Exchange 2007 connector can be scoped to only allow servers within the same Active Directory site to use the connector. Exchange 2003 will recognize all scoped connectors in other routing groups as out of scope, including any scoped connectors in the Exchange 2007 routing group. Exchange 2007 will recognize all scoped Exchange 2003 connectors and scoped Exchange 2007 connectors in other Active Directory sites as out of scope. Messages are not routed to connectors that are recognized as being out of scope.

Maximum message size

Exchange 2003 and Exchange 2007

Message size restrictions set on either server version will be applied to all messages that are routed through the connector.

Enabled and disabled property setting

Exchange 2007

Exchange 2003 does not recognize this setting and will continue to route to an Exchange 2007 connector that is disabled.

Integrated Windows authentication (formerly known as NTLM, and also known as Windows NT Challenge/Response authentication)

Exchange 2003 and Exchange 2007 Receive connectors only

Exchange 2007 Send connectors do not support Integrated Windows authentication. If you recreate the SMTP Send connectors that are configured on your Exchange 2003 bridgehead servers on Exchange 2007, you must select an alternative authentication method. For more information about the available authentication methods, see Send Connectors.

A message that is relayed from a Hub Transport server to an Exchange 2003 or Exchange 2000 computer for delivery to a recipient mailbox that is located on Exchange 2003 or Exchange 2000 must be relayed across a routing group connector. Therefore, at least one routing group connector will always separate Exchange 2007 servers from Exchange 2003 or Exchange 2000. When it determines the least cost routing path to an earlier version of Exchange Server, the routing component of the Microsoft Exchange Transport service first evaluates the possible routing paths to reach the destination Exchange 2003 or Exchange 2000 server considering only the cumulative cost of all routing group connectors that must be crossed to reach that destination. The lowest cost routing path across routing group connectors is always used, and the Active Directory IP site link cost to reach the first routing group connector is only considered when two routing paths across routing group connectors have the same cost.

Exchange 2007 routing uses the following connector selection algorithm to select a route:

  1. It selects the connector that most closely matches the address space.

  2. If there is a choice between two or more Exchange 2007 connectors, the following factors are used in the selection method:

    1. The cost of the connector. The cost is calculated by using the sum of the cost to reach one of the source transport servers of the connector and the cost of the address space. The cost to reach one of the source transport servers is zero if the source transport servers are in the local Active Directory site.

    2. Proximity of the connector. For example, the local server is closer than the local Active Directory site, which is closer than the remote Active Directory site.

    3. The connector that comes first in alphanumeric order.

  3. If the choice is between two or more Exchange 2003 connectors, the following factors are used in the selection method:

    1. The cost of the connector. The cost is calculated by using the sum of the cost to reach one of the source transport servers of the connector and the cost of the address space.

    2. The connector that comes first in alphanumeric order.

  4. If the choice is between an Exchange 2007 and an Exchange 2003 connector, the Exchange 2007 connector is always selected.

When a message is relayed from an Exchange 2003 or Exchange 2000 server to a recipient mailbox that is located on an Exchange 2007 Mailbox server, the Exchange 2003 or Exchange 2000 server considers only the cumulative cost of the routing group connectors that must be crossed to reach the Exchange 2007 routing group. The routing path that has the lowest cost is always used.

importantImportant:
The release to manufacturing (RTM) version of Exchange 2007 does not support setting a maximum message size limit on Active Directory site links or routing group connectors that include Exchange 2007 Hub Transport servers as a source or destination server. Exchange 2007 RTM also does not recognize any message size limits that are set on routing group connectors created by earlier versions of Exchange Server. Therefore, when a message that is relayed from Exchange 2007 is received by Exchange 2003 or Exchange 2000, the Exchange 2003 or Exchange 2000 server may reevaluate the routing path to accommodate message size limits. This may cause a message to be routed back to Exchange 2007, which causes a routing loop. To avoid this situation, we recommend that you remove all message size restrictions on routing group connectors when you introduce Exchange 2007 to the organization.

Exchange 2007 Service Pack 1 (SP1) provides support for configuration of a maximum message size limit on a routing group connector. By default, Exchange 2007 does not impose a maximum message size limit on messages that are relayed between Hub Transport servers and Exchange 2003 or Exchange 2000. If you use the Set-RoutingGroupConnector cmdlet to configure a maximum message size on a routing group connector, routing generates a non-delivery report (NDR) for any message that has a size larger than the maximum message size limit that is configured on any routing group connector in the least cost routing path. No alternative routing path is considered. This configuration is useful for restricting the size of messages that are sent to remote routing groups that must communicate over low-bandwidth connections. If you configure a maximum message size restriction on a routing group connector, you should also configure a larger cost for that routing group connector to prevent routing from preferring a size-restricted routing path over a routing path that has no message size restrictions. For more information, see How to Configure Message Size Limits for Internal Routing.

Exchange 2007 RTM also does not recognize a non-SMTP connector that is configured on an earlier version of Exchange Server to connect routing groups. When calculating the least cost routing path to a legacy routing group, non-SMTP connectors are not considered. Exchange 2007 SP1 includes support that lets the routing component of the Microsoft Exchange Transport service recognize non-SMTP connectors when calculating the least cost routing path.

For more information about how the least cost routing path is selected in a coexistence environment, see the "Routing Messages for Delivery to Exchange Server 2003 or Exchange 2000 Server" section in Internal Message Routing.

 
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