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The purpose of this is to support a little bit on Azure Load Balancer

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and floating IP when you're using SQL Server Always On availability

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group clusters in the cloud, okay? This is on the AZ‑801 objective so

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it's important that we cover this.

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The Azure Load Balancer is a software appliance, it's an Azure native

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resource, that provides high availability for non‑clustered hosts.

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So on the diagram at right,

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you can see coming in over the internet on TCP port 80, those service

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requests are handled by the front end of a Public Load Balancer, that's

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an Azure resource with a public internet accessible IP. We've configured

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a load balancing rule on that load balancer to forward that TCP 80

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traffic to one back‑end node that are identically configured web servers

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in this example.

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Now you'll note that the Azure Load Balancer actually can be

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deployed entirely within an Azure virtual network,

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this is the internal load balancer, and this is what's happening in the

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diagram at right with the database back end. Actually it looks like its

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Business Tier, so it's TCP 443. But I want you to know, though, that the Azure

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Load Balancer is a general purpose load balancer,

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it's happy to load balance any port. And you might remember,

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you might want to remember, that the default port for SQL

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Server is TCP 1433. If you're familiar with the Open Systems

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Interconnection reference model, the Azure Load Balancer functions at Layer 4,

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which means it makes its routing decisions based on IP

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addresses, source and destination IP address,

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source and destination port number, and protocol. Azure Load Balancer is

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normally deployed regionally, and that makes sense given that your virtual

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network and your virtual machines are all regional, but there's actually a

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global option, and this allows you to put a second public load balancer in

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front of a number of child regional load balancers. So you have some

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multi‑region load balancing going on with Azure Load Balancer, it's pretty

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cool.

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So drilling in further, the concept of the floating IP is

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this. As you can see in this example, let's just focus in

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on the DataSubnet at right,

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you've got a private IP that falls within the range of 10.200.1, that's the

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subnet range here on the DataSubnet, and so requests from web1,

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web2,

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web3 for the database will go through that load balancer's front end.

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But you might think, well wait a minute now, you said that an Always On

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availability group has a listener address,

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how do you reconcile the cluster listener IP address with the

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private IP on the front of the load balancer?

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Well that's this property that you set in the Azure Load Balancer

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called floating IP. It's just a simple checkbox control,

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it's not a Microsoft proprietary technology,

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it's actually called Direct Server Return, but Microsoft calls

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it floating IP. And at the end of the day, all you're doing is

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ensuring that that front end on the load balancer is the same IP

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address as your listener.

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Why is that important?

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Well because when the requests come into the front end of the load

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balancer and that address is the same as the listener,

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you're basically just passing through the load

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balancer directly to the listener.

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And if you do a failover, that listener is going to be updated

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with a new MAC address of which server it should then route

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traffic to. You see what I mean?

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So, long story short, that floating IP property is a real big deal

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breaker in terms of an administrator trying to get Always On

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availability group clusters available in an Azure VM.

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If you forget to set floating IP, you've got some problems. I'll show you this in our demo upcoming.