In previous articles I have dealt with the high availability of the mailbox and CAS roles. This article will now focus on the Exchange 2010 Hub Transport role.
However, the hub transport role is somewhat different to the CAS or mailbox role in terms of high availability. The Hub Transport role is actually already highly available if there is more than one Hub Transport server. As Outlook does not communicate directly with the hub transport servers but with the CAS servers, mails are not necessarily always forwarded via the same HT server. Even an Edge Transport Server that is set up in the DMZ does not route the mails via just one HT server; if there is more than one HT server, load balancing is automatically operated between the HT servers.
The situation is similar if the HT servers are configured to receive mails directly using receive connectors. If there are 2 MX records with the same priority, the sending mail server will normally contact one of the other remaining HT servers in the event of an HT server failure.
However, there is not only mail server to mail server communication but also countless applications that are supposed to send mails (also within the organization) to the mail server(s). A very simple example is backup software that sends a report by e-mail after the server has been backed up. In most cases, a DNS name or IP address is specified in the backup software to which the software should send the report/mail. This software does not rely on the DNS, so it will not attempt to deliver the mail to another mail server. Various fax server software is also configured in this way.
In this case, the only option is to use hardware load balancers or Windows NLB to keep the hub transport role highly available.
Die Konfiguration von Windows Lastenausgleich (NLB) und der Exchange Hub-Transport Rolle ist im Wesentlichen gleich mit der Konfiguration von „Exchange 2010: High availability - CAS array with Windows NLB„. Der Unterschied besteht in der Konfiguration der Portregeln wie in diesem Bild beschrieben:
If the CAS role and hub transport role are executed on the same server, port 25 (SMTP) must be specified at this point. In the DNS configuration, the MX record then points to the cluster IP address. Applications that are to send mails to the Exchange servers are then also configured to the cluster host name or the cluster IP address.