Hi.
I have the ecact same problem as Byron has described in this article : http://social.technet.microsoft.com/Forums/en-US/exchangesvrdeploy/thread/f6724caf-a7ad-47c9-af5d-1ad43459b447
I get a popup when the 2013 CAS tries to proxy OAB for a 2010 user. The only way around the problem is to sett anoteher name on the OAB url then 2013 Url. But if I do thet i have to add another Excternal IP, rule on HLB etc.
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We have recently implemented Exchange 2013 for co-existence into an environment with Exchange Server 2010. During Autodiscover, the clients are provided with the URL for Exchange 2013 CAS servers. All web services are being proxied properly except for OAB downloads.
You can see in the proxy logs you can see that it is attempting to proxy the connection to the Exchange 2010 CAS servers, but there is a 401 unauthorized error. Clients are prompted for authentication, but it is never successful.
The short term work around has been to provide access to the OAB URL via the Exchange 2010 servers but I don't think that should be required.
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Rinken
