Enabling Round Robin and Netmask Ordering

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So, what is Round Robin?  

It’s a method of load balancing that enables you to spread the network load between servers on your network.

For example, let’s say I have three WEB servers hosting the same content  with three A host records.  

All with the same name of SA but with different IP addresses.  

The goal is to load balance between these three servers to help share the network load.

That’s exactly what Round Robin will do. I’ve created another record called a CNAME record.  This record maps the alias name in this case, www to the true name sa.serveracademy.

From our DNS server. We can check the status of Round Robin and Netmask Ordering from Server Manager.

Let’s go ahead and open server manager, click tools, and DNS manager, from our server right click then click properties, then click Advanced.

And you can see that round robin and netmask ordering are both checked and enabled by default. 

  • Now I’ll show you how round robin works
  • The 1st client will get the 1st address, then the second address, then the third address. The second client will get the 2nd address first, the third address 2nd, and the 1st address third. Then the third client will get the 3rd address first the 1st address second, and the 2nd address third and down the line.  

The 4th client would get the 1st address again then back and forth and back and forth.

  • So, the advantage of Round Robin is to ensure that there is load balancing anytime duplicate names are found with different IP addresses in the DNS environment.
  • We can test Round Robin by opening a DOS prompt in administrator mode and typing nslookup, then pressing return. This puts nslookup in interactive mode.
  • if you recall that is our CNAME record and press return  
  • Now press the up arrow key and press return.

Each time you press the up arrow key, and press return  

your reloading the CNAME record, and you getting different results.  

You can see that the WEB server IP addresses are rotating, and that round-robin is working as it should.  

  • But one of the disadvantages of Round Robin is that the DNS server never goes out to see if one of the servers is down.  

So, if, for example, SA1 is down that means that 33% or more of my clients will attempt to connect and fail.  

  • So, this is not hardware load balancing,  
  • But it is there and provides this basic functionality.

Configuring Netmask Ordering 

  • One way you can enhance DNS Round Robin is by enabling NetMask Ordering
  • So, what is Netmask Ordering?  

Let’s go back to our example, Let’s say we have created one more webserver, an A record with a name of SA with an address of 10.0.1.10.

If we have one client on the 192.168.0 subnet, it will get the normal rotation of 1,2,3,4

Then the next client that shows up from the 192.168.0 subnet might get 2,3,1,4.

But if we have two more clients connect and their client IP address is on the 10.0.1.0  subnet, the SA4 server will always be the most preferred server.  

Anything in this subnet will always be on top of the list.

But these two clients on the 192.168.0 subnet will never get the address for SA4 at the top because they belong on the wrong subnet.  

For those two clients, 10.0.1.10 will always be the least preferred IP address.

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