How to configure Round Robin DNS for Exchange 2010 CAS

How to configure Round Robin DNS for Exchange 2010 CAS

Round robin DNS is a technique of load distribution, load balancing, or fault-tolerance provisioning multiple, redundant Internet Protocol service hosts, e.g., Web servers, Exchange servers, by managing the Domain Name System's (DNS) responses to address requests from client computers according to an appropriate statistical model. Round-robin works by responding to DNS requests not only with a single IP address, but a list of IP addresses of several servers that host identical services. Round Robin is cheaper and easy method a to load balance for Exchange Client Access Servers (CAS). Round robin works by assigning multiple IP addresses to the fully qualified domain name (FQDN) of a resource.

To configure round robin for this Exchange CAS, open up the DNS Management MMC and right-click on the DNS server; choose Properties. Click on the Advanced tab and check the box next to Enable Round Robin

Now drill down to the forward lookup zone and create an A(Host) record for each Exchange CAS with the same FQDN and IP address.

Alternatively, you can assign two or more IP addresses in each Exchange CAS. For example, you have two CAS servers, cas01 and cas02. Your Outlook is setup to use cas.domain.com. You can add cas IP address (10.0.5.220) to cas01 (10.0.5.215) and cas02 (10.0.5.216) LAN. To do that, open the Properties of each Exchange CAS LAN. Open the Properties of TCP/IPv4 and select Advanced. In the IP Settings, add cas IP address 10.0.5.220 to cas02.

In the Exchange CAS, the TCP/IP looks like this:

Ethernet adapter LAN:

IPv4 Address. . . . . . . . . . . : 10.0.5.216(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.0.0
IPv4 Address. . . . . . . . . . . : 10.0.5.220(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . : 10.0.0.2

In the client, nslookup shows this:
Default Server: 2008dc.domain.com
Address: 10.0.0.17

> exch-cas
Server: 2008dc.domain.com
Address: 10.0.0.17

Name: exch-cas.domain.com
Addresses: 10.0.5.216
10.0.5.215

Note: Round Robin DNS is not fault tolerant. If a user receives the IP address of a down server, they will get an error message. This is usually resolved by refreshing a few times, but they may have to go a step further and purge the local DNS cache before they get the IP address of a functional server. For this reason a low TTL value is recommended.

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