Exchange Server 2010 Coexistence : Coexistence with Exchange Server 2003

1/20/2011 11:16:14 AM
You won't be too surprised to hear that there are a lot of differences between Exchange Server 2003 and Exchange Server 2010. The most important are:
  • Exchange Server 2010 is available only in a 64-bit version.

  • Exchange Server 2010 does not use Administrative Groups for delegation of control.

  • Exchange Server 2010 does not use Routing Groups for routing messages.

  • Exchange Server 2010 does not use Link State Routing for updating the routing table.

  • Exchange Server 2010 does not use the Recipient Update Service for setting Exchange properties on recipient objects.

This is a much more extensive list than the differences with Exchange Server 2007, and the differences themselves are also more significant. Just to make sure everyone is on the same proverbial page as I go through this, I'll lay down a little background information on each of these legacy systems (e.g. Administrative Groups) before I explain what's changed.

1 64-bit support

Exchange Server 2010 is only available in a 64-bit version, as the Exchange Product Group at Microsoft is taking full advantage of the hardware advances since Exchange Server 2007 was released. The current 32-bit (X86) platform was developed in the mid-eighties, and has a 4 GB memory limit. In those days, 4 GB of memory was beyond everyone's imagination. Today, 4 GB of memory is commonly installed in a laptop.

As the successor of the 32-bit platform, one of the clear advantages of 64-bit is a theoretical memory limit of 2^64 bytes, or 16 PB (Petabytes). Windows obviously cannot address this amount of memory at this time, but the current memory limit of Windows Server 2008 R2 Enterprise is 2 TB (Terabytes). Naturally, current processors just cannot address anything like that much physical memory, but Moore's law and the inexorable march of technological progress mean that this limit will keep being pushed back in the future.

Whilst 4 GB of memory might be enough for a laptop or workstation, for large server applications like Exchange server, a mere 4 GB of memory is a huge limitation. A fact which can be clearly illustrated in Exchange Server 2003, when having more than 2,000 mailboxes on one Exchange server will result in a severe disk I/O penalty, which typically results in an expensive storage solution.

There are special techniques for addressing more than 4 GB of memory on the 32-bit platform, like Physical Address Extensions (PAE), which you can read more about here:


However, Exchange Server 2003 does not use this technique, and so is stuck with the 4 GB memory limit (and you can read more about that here: HTTP://TINYURL.COM/2003LIMIT). Given that Exchange server 2010 is 64-bit only, this automatically implies that an in-place upgrade of Exchange Server 2003 server to Exchange Server 2010 is impossible. A new Exchange Server 2010 server in a 2003 environment always needs to be installed on separate hardware. I'll mention briefly now that the same is true for Exchange Server 2007; although it is also a 64-bit application, Microsoft does not support an in-place upgrade due to technical complexity in both products.

2 Administrative Groups are no longer used for delegation of control

Exchange Server 2003 uses Administrative Groups for delegation of control, allowing you to create multiple Administrative Groups and delegate control of them to different administrators. For example, a large multinational company could create multiple Administrative Groups, one for each country, and each country could have its own Exchange administration department, responsible for maintaining their local Exchange servers. This could be achieved by delegating control of the appropriate Administrative Group to specific Universal Security Groups, which these Exchange administrators are, in turn, assigned to. This sounds pretty complicated, and after seeing such a scenario in real life, I can assure you that it is complicated. And besides being complicated, it is prone to error and I've seen it bring a world-wide deployment to its knees. It's a good thing Microsoft is not continuing this path!

Exchange Server 2010 does not use Administrative Groups any more. During installation of the first Exchange Server 2010 server, a new Administrative Group will be created in Active Directory, called "Exchange Administrative Group (FYDIBOHF23SPDLT)." All subsequent servers will be installed in this Administrative Group. Delegation of control in Exchange Server 2010 is achieved by implementing a Role Based Access (RBAC) model.

3 Routing Groups are no longer used for routing messages

For routing messages between different locations, Exchange Server 2003 uses a concept called Routing Groups. A Routing Group can be identified as a location with a high bandwidth and low latency network, such as an office with a 100 Mbps internal network where all Exchange Server 2003 servers have full access to each other all the time. When multiple locations are present, each has their own Routing Group, and each Routing Group is connected with each other using "slow links." These Routing Groups in an Exchange organization are connected using Routing Group Connectors, and so Routing Groups are very similar to Sites in Active Directory. Active Directory sites have already existed since Windows 2000 Active Directory, but Exchange Server 2003 just didn't use them and relied on their own mechanism. And to be honest, this really didn't make sense.

Instead of Routing Groups, Exchange Server 2010 now uses Active Directory sites to route messages to Exchange servers in other locations. To connect Exchange Server 2010 with an Exchange Server 2003 environment in the same Active Directory forest (and thus the same Exchange organization), a special Routing Group, called "Exchange Routing Group (DWBGZMFD01QNBJR)" will be created during the installation of the first Exchange Server 2010 server. A special Interop Routing Group Connector will also be created during the setup of that initial server, in order to route messages between Exchange Server 2003 and Exchange Server 2010.

It's also worth bearing in mind that since Exchange Server 2010 uses Active Directory sites for routing SMTP messages, every site that contains an Exchange Server 2010 Mailbox Server role will also need an Exchange Server 2010 Hub Transport Server role to be installed.

4 Link State Routing is no longer used for updating the routing table

To keep routing information up to date in Exchange Server 2003, a process called Link State is used. When a connector in Exchange Server 2003 changes its state, the Routing Table used by the Routing Group connectors is updated, and this Routing Table is sent immediately to other Exchange servers in the same Routing Group. When an Exchange Server 2003 server initiates an SMTP connection to a similar server in another Routing Group, the Routing Tables on both servers are compared and, if needed, the newer version of the Routing Table is sent to the other server.

This works fine as long as the Routing Table is not very large, but there are known cases, with over 75 Routing Groups and hundreds of Routing Group Connectors, where the Routing Table was between 750KB and 1MB in size. It might not sound like much, but when a Routing Table is being exchanged frequently, this will have a noticeable negative impact on the network traffic across the WAN.


More information regarding message routing in Exchange Server 2003 can be found in the "Exchange Server Transport and Routing Guide" which is on the Microsoft TechNet site: HTTP://TINYURL.COM/ROUTINGGUIDE. When you want to take a closer look at the routing table in your own Exchange Server 2003 environment, you can download the WinRoute tool from the Microsoft website: HTTP://TINYURL.COM/WINROUTE.

Exchange Server 2010 has replaced this whole system with Active Directory Site Links (as explained above) and thus leverages Active Directory information to determine an alternate route when a specific link is no longer available. Before installing the first Exchange Server 2010 server into an existing Exchange Server 2003 environment, Link State Updates need to be suppressed to avoid routing conflicts between the Exchange versions.

5 Recipient Update Service versus Email Address Policies

The Recipient Update Service (RUS) in Exchange Server 2003 is the service that is responsible for updating the Exchange specific properties of Exchange recipients in Active Directory. When a user is created with Active Directory Users and Computers, the RUS will pick up the user account and "stamp" it with Exchange specific attributes, such as the homeserver, homeMTA, homeMDB and email addresses. It can take some time for a user to be fully provisioned and available, especially on busy servers. The Service is part of the System Attendant, and only one instance is running in each Active Directory domain.

Exchange Server 2010 does not use the Recipient Update Service anymore, but uses Email Address Policies instead. When a mailbox-enabled user is created, an Email Address Policy is applied immediately, and the mailbox is therefore available instantly, though of course the user object needs to be replicated between all Domain Controllers in your environment to be fully available at all locations. In a coexistence scenario, the Recipient Update Service and the accompanying Recipient Policies can only be managed from the Exchange Server 2003 System Manager, and the Exchange Server 2010 Address List Policies can only be managed from the Exchange Management Console or the Exchange Management Shell. The only time a Recipient Policy is accessed using the Exchange Management Shell is when upgrading the Recipient Policy to an Email Address Policy.

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