Let us Start with Some basic understanding:

Q1) What are Routing Groups

A1)Exchange 2003 use routing groups to define an Exchange-specific routing topology. Typically, routing groups are used to specify a set of well-connected Exchange servers. Servers in the same routing group can communicate with each other without the use of connectors

Q2)What do we need to build Routing Group Connectors ?

A2)When more than one routing group is defined in an Exchange 2003 organization, you must manually create routing group connectors to enable mail flow between Exchange 2003 servers in different routing groups.The routing group connector must specify a source server and a target server as the connector endpoints. A routing group connector defines a one-way connection, and a reciprocal connector must be created to establish mail flow in both directions. The source and target servers are the bridgehead servers for the routing group.

Q3)What is The Email Routing mechanism in exchange 2010 ?

A3)In Exchange 2010, you don’t have to define an Exchange-specific routing configuration. Exchange 2010 uses the existing Active Directory site topology to define its routing topology.Mail routed to Exchange servers located in different sites must be relayed by Hub Transport servers. Hub Transport servers send E-mail to Hub Transport servers in remote sites by using the intra-organization Send connector. The intra-organization Send connector is an implicit connector computed by using Active Directory site and IP site link information

Summary:

If you have read the above Two Paragraphs you would be like

“Hmm so for Integration between these two Topologies Because Exchange 2003 is the legacy technology so Exchange 2010 When is installed Needs to be placed in a Pre Configured Routing group Which should be Made automatically during the Installation of Exchange 2010”   RITE ?

Lets Build on to our Understanding:

To support coexistence between these two routing topologies, all Exchange 2010 servers are automatically added to a single routing group when Exchange 2010 is installed. The Exchange 2010 routing group is recognized in Exchange System Manager in Exchange 2003 as Exchange Routing Group (DWBGZMFD01QNBJR) within Exchange Administrative Group (FYDIBOHF23SPDLT).

This is the Question Which we are Asked Everyday on Microsoft Forums by people

Question : During the Exchange 2010 Installation in an origination which already has Exchange 2003 Installed which server do we have to Enter during the Exchange 2010 Installation

Answer:“During the installation of the first Exchange 2010 Hub Transport server in an existing Exchange organization, you must specify an Exchange 2003 bridgehead server to which to establish the first routing group connector. We recommend that you select a bridgehead server located in a hub routing group or in a routing group that has many mailboxes. The routing group connector links the routing group where the Exchange 2003 server resides and the Exchange 2010 routing group. The Exchange 2010 routing group includes all Exchange 2010 servers, regardless of the Active Directory site in which they reside”

Note: Now What the Above Step Does is

Exchange 2010 is one routing group connector and the Bridgehead Server you defined in the Above Step is in one Routing Group Connector.

Now all The Emails Flowing from Exchange 2003 To Exchange 2010 Would Be VIA these Connectors. So If you want to Create Additional Connectors So that Different routing Group can Directly Communicate With Exchange 2010 Routing group So we have to Build Connectors Using

Set-RoutingGroupConnector cmdlet in the Exchange Management Shell. It’s a best practice to specify more than one source server and more than one target server to provide redundancy and server availability.

Caution:

Don’t move Exchange 2010 servers out of Exchange Routing Group (DWBGZMFD01QNBJR), and don’t rename Exchange Routing Group (DWBGZMFD01QNBJR) by using a low-level directory editor. Neither action is supported. Exchange 2010 must use this routing group for communication with Exchange 2003

Important:

Placing Exchange 2010 servers and Exchange 2003 servers in the same routing group isn’t supported.

If your existing Exchange environment includes more than one routing group, you may want to create additional connection points between Exchange 2003 and Exchange 2010 to optimize mail flow. To create additional connection points, follow these steps:

  1. Determine how you will upgrade the organization to Exchange 2010. The order in which you decommission routing groups will determine which Exchange 2003 routing groups should connect directly with Exchange 2010.
  2. Modify the registry to suppress minor link state updates on all the Exchange 2003 servers. This configuration change prevents connector state messages from being relayed throughout the organization by using link state updates, but doesn’t prevent configuration change messages from being relayed. For more information, see Suppress Link State Updates.
  3. Use the New-RoutingGroupConnector cmdlet in the Shell to create all routing group connectors that specify Exchange 2010 Hub Transport servers as source or target servers. Configure a routing group connector from the Exchange Routing Group (DWBGZMFD01QNBJR) to each Exchange 2003 routing group with which Exchange 2010 will communicate directly, and configure the corresponding reciprocal routing group connectors. You can use the Bidirectional parameter with the New-RoutingGroupConnector cmdlet to create both connectors in a single operation. These connectors will enable mail flow between Exchange 2003 and Exchange 2010.

After reading the Above Paragraph the Question in Your Mind would be

What are LINK State Updates and why do we need to suppress minor link state updates on all Exchange 2003 Servers.

Answer

Part1)Exchange 2003 servers maintain a link state routing table that’s updated through communication with the routing group master. Each connector that has been created between Exchange 2003 routing groups is considered a link. Exchange 2003 servers determine how a message is routed inside the organization by using the cost assigned to these links. If a particular routing group is inaccessible by using the lowest cost route, the link state table is updated by the routing group master to show the state of that link as down. This data is communicated to every routing group in the Exchange organization

Part2If multiple paths exist between the Exchange 2010 routing group and any Exchange 2003 routing group, minor link state updates must be suppressed to make sure that message looping doesn’t occur when a route is recalculated. It is recommend that minor link state updates be suppressed for each server in the Exchange 2003 organization. When link state updates are suppressed, Exchange 2003 servers also queue at point of failure, instead of recalculating the route.

“”Link state routing isn’t used by Exchange 2010 Hub Transport servers. Exchange 2010 can’t propagate link state updates, and it doesn’t recalculate routes. Hub Transport servers always try to communicate directly with other Hub Transport servers. When a connection to a site is unavailable, Exchange 2010 uses the IP site link costs associated with Active Directory sites to determine the closest site at which to queue the message. This behavior is known as queue at point of failure. The message queue generated at the point of failure is put in a retry state””

Hope this Post was helpful In understanding the Routing topology in Exchange 2003 and Exchange 2010

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