NLB eliminates a single point of failure if only one server is configured. It also allows for spreading the network load over multiple servers to prevent overloading one server.
They are called an NLB Cluster.
Each cluster member is assigned a unique IP address. The cluster itself is assigned a single shared IP address which is used by clients to make requests.
Servers use a Heartbeat signal every second to maintain consistent cluster membership.
You can configure Unicast mode where the MAC addresses used by cluster hosts are replaced by a single cluster MAC address. Cluster hosts require a second NIC to communicate with each other.
You can configure Multicast mode where a common multicast MAC address is used by cluster hosts. Cluster hosts are able to communicate with each other so additional NICs are not available.
You must install the necessary services such as IIS for web traffic and be running the same version and edition of Windows Server. You must configure static IP addresses on each server. Optionally, you can install a second NIC to allow communication between individual servers in the cluster.
Port Rules are used to identify the cluster IP address and traffic characteristics such as port number, protocol, and how the traffic is handled by the cluster.
Failover Clustering is a server feature that increases server availability and fault tolerance. This type of clustering uses shared storage resources unlike NLB and migrates services on failed servers to available servers in the cluster.
Failover Clustering uses the concept of a quorum to identify when the cluster can continue to run and when it must stop due to a failure in one(or more) of its members.
Methods for determining whether to continue or stop running include:
Node Majority (if more than half of the servers are still running – usually when there is an odd number of servers in the cluster),
Node and Disk Majority (uses a witness disk to keep a copy of cluster configuration information – if a majority of servers is available in addition to the witness disk then the cluster keeps operating),
Node and File Share Majority (the witness disk is stored on a share instead of one of the cluster servers), Cloud Witness (uses the cloud for storing the witness information),
No Majority (as long as one server and the witness disk is running), and Dynamic Quorum (the cluster is dynamically managed using vote assignments based on the state of each node.
Other important features include: Scale-Out File Server Support, Cluster-Aware Updating, Virtual Machine Monitoring and Management, CSV Enhancements, Guest Clustering, and Active Directory-Detached Cluster support.
Hardware
Software
Cluster storage – if implementing a SAN this should only be used for the shared storage.
Cluster Node Communication – use a separate network for the cluster node heartbeat signals.
Client Connections – this network is used by clients to connect to clustered resources and services.
A multi-site cluster is a geographically-dispersed cluster that contain nodes in multiple sites. They are typically connected using WAN links.
Automatic updates can trigger a failover. You can configure Cluster-Aware updating (CAU) to avoid triggering a failover.
DFS Namespace Server DHCP Server File Server Hyper-V Replica Broker Internet Storage Name Service (iSNS) server Network File System Virtual Machines WINS Server
Hyper-V Replica allows Hyper-V virtual machines to be replicated from one Hyper-V server to another, which provides fault tolerance.
Physical Location - can be in the same physical location or geographically dispersed.
Network – must have sufficient network BW to allow for replication traffic.
Storage HW – shared storage or cluster technologies are not required but each server must have sufficient storage for the VM to be replicated.
Server – must both be running the same version of Windows server.
Domain Membership – replication is easier if both machines are in the same domain but not required.
Encryption – replicated data can be encrypted using certificate-based authentication.
Create the virtual machines.
Configure the Replica Server to Accept Replication
Enable Virtual Machine Replication
Monitor the Replication
Storage Replica allows you to configure block-level replication between either Windows servers or Windows clusters for disaster recovery. It can also be used to stretch a failover cluster across multiple sites to ensure high availiability.
Storage Replica types include
Synchronous Replication where applications running on a Windows server can immediately write data to servers at two or more physical locations at the same time and then acknowledge the disk write operations before being marked as complete by the local server.
With Asynchronous Replication the write operations can occur at the same time, but the acknowledgement between the local hard disk and the replication partner may occur later. This can have better performance, but the risk of lost data if the network fails before the data is replicated properly.
Stand-alone configuration where two Windows servers are used as replication partners
Stretch Cluster where storage replication occurs between sites.
SSD allows a single storage pool to function as shared storage within a Windows cluster.
This allows direct-attached storage to create the storage pool which are relatively inexpensive.