How Free/busy information will flow in a Exchange mixed mode (Exchange 2007/2003)

The below procedure explains the steps involved in fetching free busy information of E2K3 mailbox from Exchange 2007 environment.

1. User initiates a Meeting Request using Outlook 2007 and adds the Exchange 2003 recipient as attendee
   
2. CAS is responsible for fetching the free/busy information, so Outlook waits for CAS response
   
3. CAS queries active directory for E2K3 user legacyExchangeDN attribute and also queries for the list of Exchange 2003/2000 servers in the AG to which legacyExchangeDN value points to.
   
4. CAS forms a http query for free busy information and passes that to one of the servers it identified in AG. There is no specific order AFAIK, it does the selection randomly.
   
5. If it gets a response, it passes that to Outlook and outlook displays the information for user.
   
6. If CAS didn't gets any response for its HTTP query, it makes the query to another server in AG and continues with remaining list till finds a server which responds to the query
   
7. If none of the servers in AG are responding to the HTTP query, it logs an event in event viewer (event id: 4004 and Event Category: Availability Service) and tries to pass the query to a random server in Exchange Organization as a last attempt.
   
8. If the CAS gets response at last attempt(as said in step 7), it passes that information to Outlook
   
9. You will see gray lines in Meeting request scheduling Tab, if CAS is unable to get Free/Busy information of E2K3 user from any of the servers which are in AG and if the last attempt to random server also fails.
   

Because of this behavior, MS recommends adding all the servers in a AG to free/busy public folder of that AG. If your organization is not meeting this criterion, then you will see availability related errors (event id 4003) in your CAS event log.

Please note that, above procedure is valid only for meeting requests initiated from Outlook 2007 using Exchange 2007 mailbox to Exchange 2003 mailboxes. If your scenario is not same, then the procedure will change. I will come-up with details about this in my next post. So, stay tuned J

Happy Learning,
Sitaram Pamarthi